Links to sites and ressources I find useful. For the Abandonware links, just know that the whole concept of Abandonware is a legal grey zone.

Downloads:

Abandonware DOS is an ever-expanding archive of classic PC games originally released for DOS, Windows, and Macintosh. A database of games that date back to the 80s and 90s, and they are available for download. With Abandonware DOS, you can fully satisfy your nostalgia for retro gaming

DosGames.com: The purpose for this website is to provide a source to download free (shareware, freeware, public domain, etc) and legal (no warez or abandonware) DOS games. Most (if not all?) of them can even be played directly in your browser.

Internet Archive: The Internet Archive Software Collection is the largest vintage and historical software library in the world, providing instant access to millions of programs, CD-ROM images, documentation and multimedia.

Win31.de: The site for Windows 3.1x with Downloads, Links, Tips and more...

WinWorld is an online museum dedicated to the preservation and sharing of vintage, abandoned, and pre-release software. They offer information, media and downloads for a wide variety of computers and operating systems.