phantasy star portable 2(skip the first one) not much of a sequel story wise, and alot of the stuff from 1 makes an appearance in 2. 2 was a huge improvement over 1 though gameplay and content wise. plus it has online multiplayer whereas the first one was only ad-hoc. there might be a demo for this avaiable still too. this is also still on sale for 15.99 on psn til the next update.
monster hunter is also good, but a bit grindy.
metal gear peace walker - easily one of the best games on the psp. there is a demo available.
kingdom hearts, disgaea 2, and final fantasy dissdia are all great games with alot of content.
cladun: this is an rpg - only available on the psn as far as i know. this game has 8-bit graphics, but there's alot of content. there's also an in-game sprite maker. it's mainly for heads, but you can make alot of neat stuff off of it.