Quake (official demo download), id's 1996 sequel to Doom, was more than just another FPS best seller on the PC. It was the game that cemented the dominance of 3D graphics, pioneered the keyboard & mouse interface, single-handedly exploded the 3D graphic-cards market, introduced mankind to online fragging and made sure zombies would be our collective FPS enemies for many years to come. What's more, its historically important engine has gone open source since 1999, which eventually led to the creation of the Dark Places Quake mod software thingy.
Why should you care? Simple really, as a) you are all caring by nature and b) Dark Places can give the aging Quake quite a facelift. The mod, you see, provides Quake with a vastly improved engine sporting (among other impressive features) advanced lightning, shader effects, 32bit color alpha blended explosions, improved physics and some really nice particle tricks. To grab said mod click here. Oh, and don't worry, it's freeware exactly as the gods intended.
Now, to further enhance your retro-ish FPS experience, you could also give the Quake Retexturing Project and Quake Remodelling Project a try. They too are free and quite impressive.
Related @ Gnome's Lair: Bioshock widescreen hack & stuff, Duke Nukem 3D high-res, fixing the RE4 PC mess, Warcraft III widescreen
