That's a heavy topic. Do we only count classic releases or even if it was released way later on PCC?
And from some of the ones already mentioned here a lot of them had their entire Franchise released on both.
Final Fantasy 7,8,9
Tomb Raider 1,2,3,4,5
Resident Evil 1,2,3
Metal Gear Solid
Tony Hawk's Pro Skater 2,3,4
GTA 1,2,London
Re-Volt
Quake 2 (though the PS1 Port has altered Levels)
Forsaken
Tom Clancy's Rainbow Six & Rogue Spear
Rayman 1,2
MDK
Hidden & Dangerous
Test Drive 4,5,6
Shadowman
TOCA 1,2
Driver
Theme Park & Theme Park World
V-Rally 1,2
Colin McRae Rally 1,2
Command & Conquer, Red Alert and at least one of the Expansions
Dino Crisis
G-Police
Need for Speed 1,2,3,4,5
pretty much all of the Licensed EA Sport Games (FIFA, Madden, NHL, NBA)
pretty much all of the Licensed EA Racing Games (F1, NASCAR)
Doom (though the PS1 version doesn't have all Levels, some are altered and it has most of Doom 2's content as well)
Final Doom
Wipeout & 2097 / XL
Wing Commander 3,4
Destruction Derby 1,2
NASCAR Racing (the one from Papyrus)
Urban Chaos
South Park
Croc 1,2
Castrol Honda Superbike 2000
Racing Simulation 2
F1 World Grand Prix
Duke Nukem 3D (PS1 as Duke Nukem in EU and Duke Nukem: Total Meltdown in US)
And that is just what came from the top of my head, I bet that is just scratching the tip of the ice berg. Baldur's Gate (mentioned above) wasn't actually released on PS1, but there is a very playable Beta of it avaiable.