At the moment, we already have 11 FPS games on here:
Alien Trilogy
Blood
Operation Body Count
Descent
Deus
Doom 2
Ken's Labyrinth
Montezuma's Return
Strife
Terminal Velocity
Wolfenstein 3D
Then there's arguably these, too:
Elite Plus
Mechwarrior
MDK
Wing Commander
XWing
There're still more coming in, too, as there're still quite some FPSes out there that're lacking on the site (like Entombed, for example)