Resident Evil 4 was added to the Humble Capcom bundle today 
some other games that come to my mind:
Beyond Good & Evil
Devil May Cry 3 (but the port is pure shit)
Deus Ex
GTA III, VC and SA
Half-Life 2 (was on the original Xbox)
Max Payne 1+2
MDK 1+2
Prince of Persia: The Sands of Time
Quake III Arena (or simply get Quake Live for free)
Splinter Cell: Chaos Theory
Star Wars: Jedi Knight 2
Star Wars: Knights of the Old Republic
Unreal Tournament
Worms Armageddon
And some games are free nowadays (outside of Steam) like Command & Conquer 1-3, GTA 1+2 or Warzone 2100 (one of my favourite PS1 games).
Not that many PS1 games because multiplats weren't as common as today. And publishers are probably hesitant to re-release their old PS1 -> PC ports because most of them were garbage.
