Virtua Fighter 5 if you like fighters... this excluding smash bros is the best fighting game out there period.
Oblivian if you don't have an earlier version.
Motorstorm if you like arcade racing.(not pure driving simulation) - forget the demo, it sucks compared to the real game. The real game is great.
download gripshift $10 off the PSN store, well worth the cash.
maybe pile, if these your kind of games and you don't have for another system:
Godfather: GTA clone type game.
Enchanted Arms: J-RPG, annoying voices, cheesy dialog, ext... but a good rpg fix.
Fight Night round 3: If you're a boxing fan.
Armored Core 4: If you're into mechs and have a LOT of time to learn a game where the mech customization system is really freaking complicated.
Import Gundam Musou: If you like Mechs but don't want to spend 2 hours learning how to customize properly. - There's a demo out so you can try.
If you like baseball... DO NOT GET MLB 2k7! Check out the demo first!!! Personally I think you're better off waiting a week and getting MLB 07: The Show. My best friend IRL is actually waiting on buying a PS3 til the Show comes out.
Get Resistance obviously... you'll love it. I may suggest as well... F.E.A.R. if you're info FPS, but that's based on what people are saying, I haven't personally played it yet.(I ordered it online but I had it sent to my house and I'm still at my dorm.) I'm assuming of course you don't have it already for PC or 360.
Unless you already have an opinon of Tekken vs Virtua Fighter, I suggest you just get Virtua Fighter and don't bother buying Tekken 5: DA for $20 or MK2 for $5. Virtua Fighter is ALL you really need... unless you're already a Tekken or MK2 fan...
And my friend all love Virtua Tennis - personally I think it's kind of simple and boring.
http://www.qcdn.org/ffxi/showthread.php?t=43249
And that link is potentially useful. I might wait on Linux though until they get a distro working that taps into the PS3's full power. When that happens I'm sure those linux dudes will pump out one based on using your PS3 as a Tivo and to run all the retro system's emulators. But if you have any interest in that eventually, you might as well partition 10 of your gigs so you don't need to format later and have to backup your save files.