Currently everybody is waiting for Duke Nukem Forever to be released. Some people (like me) wait for this since 1997.. you ask why? Because Duke Nukem 3D showed us what it means to kick some alien asses.
Duke Nukem 3D is a FPS developed by 3D Realms and was released 1996. There exist Duke Nukem and Duke Nukem II, but they were pretty lame 2D Jump&Run games.
You have a great repertory of weapons including some futuristic weapons like the Shrinker or Freezing Cannon.
What made Duke Nukem 3D so great weren't only very good graphics (we're talking about 1996 here) compared to Doom, Hexen and other FPS these times, but the quotes Duke speaks out from time to time and other funny possibilities (e.g. Having strippers open the bra for money, lighten yourself at the toilet, etc.)
I would recommend everyone who is going to play "Duke Nukem Forever" to take a look in it's previous alien-ass-kicking-fps. You won't regret it.
PS: I'm currently playing the Atomic Version of Duke Nukem 3D, which also works on Windows 7 and has an additional Chapter :D