Shameless repost, but anyway
I've always been a retro-gamer of sorts, and I'm glad to see that old game sites and forums are quite active these days. Here's a list of a few noteworthy ones - even if you haven't been inclined towards old games before, you might just as well try going retro now
Freeware & Shareware Archives
Remain in Play
This is as much a database of commercial games that have been released into public domain as it is a gaming site. It provides info, screenshots and downloads of freeware games, but keep in mind that the database includes not only playable games, but also source code and raw data if those were the only things released to the public. There options to filter games by type of distributed content, and it's a good idea to look for binary executables if you want to get playable games and not files that are barely useful if you aren't a programmer or game developer yourself.
Theodor Lauppert's Game Gallery (mirror site)
Here you'll find interesting, insightful reviews of games, tons of related links, screenshots, demo/shareware/freeware downloads etc. You can browse games by genre, title, platform (featured games aren't limited to PC only), or country. There are also a few interesting related sites:
King Svatopluk's Court - a nice site dedicated to various RPGs like the Elder Scrolls series.
Svatopluk's Game World - various articles about games.
Svatopluk's DOSBox - a large, unsorted collection of screenshots from various DOS games.
Svatopluk's Arcade - a lot of info and screenshots about all sorts of arcade games.
Demu.org (formerly known as DOS Museum)
This is by far the largest archive of shareware episodes, demo versions, extras and utils for DOS and Windows games, as well as a selection of other programmes. Quite a few rare items can be found here, so it's a must see for everyone The only imaginable drawback is that original file names of archives aren't always preserved, and some games have been repackaged in RAR for better compression. Both playable and unplayable demo versions (and also preview slideshows, trailers etc.) are present. There's an option of ordering a DVD with the entire collection of the site per mail. Links to sites that offer full versions of featured games for purchase are present where applicable.
RGB Classic Games
Behind this site is an extensive agenda of preservation of old games (DOS and Windows), and thanks to the initiative of the site's author some old titles have been officially released into public domain by their respective developers following Internet petitions. RGB Classic Games is unique because it aims to include every existing version of each featured game, and the community members actively search for rare releases of old games. Each game on the site is accompanied by an brief but informative description that contains info on developers, publishers, current availability etc. Non-playable content is usually not included. The site also offers their complete collection on DVDs. Links to sites that offer full versions of featured games for purchase are present where applicable. There's also an extensive archive of emulators and other utilities you might need to play the games on modern machines, and comprehensive guides to successfully running old games on new hardware. In addition, there is an option to play featured games right in your browser.
Hallfiry's CD Index - an incredibly powerful search engine that allows to view contents of more than two thousand magazine cover disks from the 90s and the 2000s. Prominent English, German, French, Polish and other magazines are featured, and links to CD images are present where possible (including links to the Internet Archive's CD Archive).
Kult Cover Disks - The Cover CD & Magazine Archive! - Hallfiry's extensive collection of gaming magazines and magazine cover disks (mostly in German).
Kult Game Downloads - Hallfiry's database of shareware and demo releases of PC games from the 90s and the 2000s, as well as patches and various extras.
Smush The Cat
A nice collection of shareware episodes, demo versions and freeware full versions of old DOS and Windows games. Descriptions and other info are scarce, but links to official and related websites for games are present if possible. There's an option to browse by multiple categories (platform, game type etc.). The site also offers utilities that might be needed to run the old games on modern systems.
This is a popular site with a large archive of DOS games, offering playable demo versions and shareware episodes. Well-known classics are present alongside more obscure titles. There's also a community forum where people can get help with getting a game to run on a modern machine, or locating an old game they just can't find. A selection of utilities needed to run the games is also available.
DOS Games Archive
A sister site to dosgames.com. Non-playable content is also present, as are full versions of games that have officially become freeware. There's a multi-category browsing option (by genre, playability etc.), a number of selected screenshots for each game (unfortunately, they're not always from the demo version), cheat code listings and links to places where you can buy the full version if a game is still sold.
The DOS Multiplayer Archives
This site focuses on playing old DOS games over the net (through DOSBox). It offers demos of DOS games that have multiplayer capability, already bundled with DOSBox configured for online play. The collection of games isn't very large at the moment, but it can be expected to grow. Currently, the focus is on the less known games that have a multiplayer option, because they're not as easy to run on modern systems as those well-known classics (Doom, Duke Nukem 3D etc.) that have source ports to newer platforms, as well as large supporting communities.
Hardcore Gaming 101
A dedicated old-gaming site with a huge database of detailed articles about PC and console games and game series.
The Computer Show
This is an archive of previews, reviews, walkthroughs and other articles about games that have been released in the nineties.
Ancient DOS Games
Periodic issues of video reviews of DOS games can be found here.
HappyPuppy (archived copy)
This seems to have been one of the most active gaming sites in the nineties, with reviews and links to shareware and demo downloads. Definitely worth of checking it out!
Gamers.Org has several old FTP file archives on their site:
FTP Archive at the Polish Interdisciplinary Centre for Mathematic and Computer Modelling
A nice FTP archive of shareware and demo programmes and games.
The following are mirrors of a certain old FTP site with shareware and demo downloads:
Sites Dedicated to Specific Games and Companies
3D Realms' Master Download Page
Apogee Software, more commonly known as 3D Realms after it's sub-brand focused on 3D games, pioneered the shareware model of game distribution, in which a sizeable portion of a game ("episode") was offered for free download and distribution, and the rest could be ordered by "registering" the game, if the player liked it. This successful strategy was later employed by other companies, like id Software, and the result is a large number of classic shareware games that are freely distributed. The downloads archive features shareware episodes of games published by Apogee/3D Realms, and also offers full versions of games that have become freeware. Generally, the 3D Realms website has a lot interesting stuff to explore, visit their FAQs section and the extensive Apogee Legacy interviews archive by Joe Siegler.
El Zee's Amulets & Armor Forum
A fan forum for discussing Amulets and Armor, a first-person action/RPG game that runs on the Doom engine.
leileilol's Unofficial Amulets & Armor Shrine
A little site about the game, which aslo includes downloads of the shareware version.
Rise of the Triad Headquarters
As the name suggests, this is a site dedicated to Apogee's Rise of the Triad, and it contains extensive information about the game and everything related to it. The downloads section offers source ports, the shareware version, and various extras. There's also a forum.
The Sierra Help Pages
As the name suggests, the site is dedicated to games by Sierra On-line. There's extensive information about the games, hints and tips, and helpful information about running them on modern machines. The site also features and extensive collection of demo versions of Sierra games.
The Sierra Chest
An extensive database of Sierra games, with screenshots, downloadable demos, and various links.
SierraGamers - The Official Website of Ken and Roberta Williams
Exactly what it says on the tin, it's a site maintained by Ken and Roberta Williams and dedicated to Sierra games and legacy. A must visit for every Sierra fan, and anyone interested in the history of computer games.
The Chaos Regime
A site about games by Bitmap Brothers. Features demo downloads, screenshots and interesting info.
Interplay Official Website
The official website of Interplay Entertainment. They've got tech support for their old games, as well as demos, patches and other extras like PDF manuals and, in some cases, even music. Some links seem to be broken though.
A tribute to Westwood Studios, this archive at CnC World is a copy of the company’s official site from 1996.
Miscellaneous Gaming Sites
A Force for Good
This website is about old PC games, and how to run them on modern systems. It offers detailed reviews of many games, both for DOS and Win9x, and general information articles about where to find old games, and what tools and tricks to use to get them running.
Videogame Music Preservation Foundation
A database of music from video games, regardless of platform. You can browse entries by games, by artists, formats or platforms. Also has its own Wiki.
leileilol's 90's Hunter
An interesting collection of obscure shareware and freeware games from the nineties. Also featured are largely unknown modifications for certain old games.
The site features a rather extensive list of old FPS games, organized by year of release, and contains screenshots and download links for demo/shareware/freeware releases of each game (if applicable). The content seems to exclude vehicle FPS games though, but it has quite a few rare titles so it's worth looking into.
A neat site about oldies, with various interesting and useful info about classic games (and, in some cases, utilities like game data extractors).
CuteFloor’s YouTube Channel
Has a large section of videos from unreleased DOS games, as well as alpha and beta versions.
Retro Game Forum
A promising forum dedicated to old gaming on various platforms, including home computer systems, consoles and game arcade machines.