Duke Nukem 3D (1996): This was the first person shooter that was  to be the prelude to the infamous Duke Nukem Forever and remains a competent game today. Not as technologically accomplished as its rival at the time, Quake, Duke 3D stood out on its over-the-top characters and fun weapons. Xbox Live has a fairly well done port for download for $10, and fans have continued work with the PC version. The Xbox Live version offers a traditional demo, while the PC version remains shareware. I seem to recall some free versions of it floating around on old PC Gamer CDs several years ago, but I’ll be darned if I can actually find one. In any case, the Xbox Live version is linked below. The PC version honestly requires some research, so I’m pointing you to its Wikipedia entry.

Xbox Live Duke 3D
PC Duke 3D

Terminal Velocity (1995): A flight ’sim’ of sorts, this game put you in control of a sci-fi starfighter. You could fly through a series of fairly large (for the time) worlds, fighting all kinds of enemies with a very flexible flight engine and lots of power-ups. While not realistic by any means, the controls were nonetheless fun. Terminal Velocity remains shareware and can be downloaded from...

Terminal Velocity: http://www.3drealms.com/tv/index.html 

For space concerns, I couldn’t include every game they’ve published that was worth playing. For example, they had much to do with the original Wolfenstein 3D, but it didn’t really fit the article’s theme; nor did Death Rally. I’ve mostly limited the games linked above to ones I distinctly recall enjoying, and I hope you’ll have fun with them too. If you take away anything from this, I hope it’s an appreciation for the classics they made. While the foolishness of Duke Nukem Forever is a powerful lesson in how to ruin a company, it should not be the only legacy they leave us.

For those of you curious about the actual history of Apogee/3D Realms from its earliest days to about 2005, there is this wonderfully researched guide on them: http://rinkworks.com/apogee/ or directly from the 3DRealms site.