http://www.3drealms.com/downloads.html
This was actually a Christmas happening, but it's not too late to check it out. 3D Realms added the classic sidescrolling platformer Bio Menace to their growing back catalog of Apogee freeware. It's surprisingly very good, with some of that Duke Nukem I/II feeling but totally original graphics. Aside from the music (Bobby Prince, great musician) and perhaps some other minor work, the programming, graphics, and (I think) even the box art were all done by one guy.
There's a news post about the game on 3D Realm's front page; can't miss it.
I haven't played too much of the game yet as the soundcard emulator I'm using limits playtimes to 4 minutes, but so far it's been pretty amazing. Remember the original Duke Nukem, with its stolen Turrican platforms? No more; there's a huge variety of tilesets and unique graphics for monsters and such. The game's excitement level is a bit limited as the character is pretty limited in movement options and enemies are pretty static as well, but the environments really make it worth blasting your way through the levels.
There's some tips for running the game in DOSbox, but I got the game to run just fine without any modification on my other Windows XP machine by using a soundcard emulator called "SoundFX2000." It does Sound Blaster 16 and AWE 32, among other things. It's an awesome program, but you need to register it or else you'll only be able to play your games for 4 minutes. Registration isn't cheap, either; it's $39 dollars. Price aside, it seems to be superior in resource usage, compatibility, and sound quality to VDM Sound, which I had a heck of a time with. Of course, if you have an older box, you can just put the files on a few floppies and it should work just fine.