I've not touched mine since a month after i bought it. Honestly it's the worst handheld I've ever owned. To be fair I've only owned nintendo handhelds but still..thing has like 3-5 hour battery life..wtf. 
 
Rant over n all that. Theres the new MGS out for it. Beyond that..uh..china town wars?

P3P is great.
 
Patapon 2 is awesome. 
 
I don't know, there's some more, but overall I agree about the PSP leaving some to be desired.

" P3P is great. Patapon 2 is awesome.  I don't know, there's some more, but overall I agree about the PSP leaving some to be desired. "

" P3P is great. Patapon 2 is awesome.  I don't know, there's some more, but overall I agree about the PSP leaving some to be desired. "

I make it a point to touch my PSP on a daily basis.

I hate the psp. I haven't touched mine for 1 1/2 half years, just been sitting in my closet. It's the original PSP fat, so it got some use before I locked it away.

I still play the DJ Max games from time to time, but that is all it is ever really good for anymore. I already have Persona 3 FES, so it's disappointing to know that the only good game that has come out in a while is one that can already be played on a PS2.

"I haven't touched my psp since early 2009."

Persona 3 Portable is the only game I'm thinking about for PSP.  
 
Lumines, Metal Gear Ac!d, Patapon, and Loco Roco are the only games that I ever really truly enjoyed on the PSP. Their sequels included. And I enjoyed the living hell out of Lumines.

Ace Combat Skies of Deception is pretty good imo as well...

I just played the mini "Monsters (probably) Stole My Princess".  It's on sale this week for like $2.50.  It's short, but has replay value if you wanna get high scores and whatnot.  I highly recommend it, especially while its on sale.  Echoshift is also a game on sale this week for like $7.50 and that's a pretty cool puzzle game.  If you haven't played God of War: Chains of Olympus, you should pick that up.  Little Big Planet is also a stong title on the PSP.

Off the toppa my head... Crush, Persona 3 Portable, Powerstone Collection, Persona Remake, Peace Walker/Portable Ops, Metal Gear Acid, Patapon, Loco Roco, LittleBigPlanet, ModNation Racers, Castlevania Dracula X Chronicles, Valkyria Chronicles 2, Fat Princess, Gran Turismo, Metal Slug Collection, FF7: Crisis Core. There's some other good ones yet to come out like Resident Evil Portable, the new Kingdom Hearts if you're into that I suppose, new Parasite Eve, new God of War.  Still some life in the PSP yet.

I recently bought one cause it was only 50 bucks.
Let's just say I'm currently happy it was only 50 bucks.
 
I've seen a lot of people defend it over the years, funny thing is when you go looking for information on PSP games (on this site and others) you can't help but feel that very few people actually play this thing.
 
I'm sure as I keep searching I'll discover some gems at some point but so far most of the "big name" games haven't done it for me, and everything else is pretty lacking in the demos and information departments.