I'm going to submit a few of my favourite forgotten FPS games:
Half-Life 2
Doom 3
FEAR
See what I did there? Eh? Christ, what's wrong with you people!? Duke Nukem? AvP? Turok? These games are NOT forgotten! They've all either had sequels made for them, or currently have sequels in the making - can you please explain to me how that qualifies as forgotten? This thread has so much potential to be a brilliant trip down memory lane, but some people have some whacked definitions...
I do however remember when that Terminator FPS was released WAY back in the day. My fondest memory of that is when you shot the moon and it died
-RaZ