The rather fine British Gaming Blog has an in-depth article on the homebrew PSP emulation scene, specifically related to classic PC games.

Some of the titles referenced, for those who have the correct OS and know what to do, include the classic Duke Nukem 3D ("The PSP port plays well with a good framerate and comprehensive controls; there is no support for the numerous expansion packs, however"), and Quake 2 ("The PSP port is still a beta and has some issues with control and sound, but the team are still working hard at getting it to be one of the most impressive PSP homebrew titles out there.")

The piece ends with some wished-for titles: "Some titles that need to be ported? I’m not a homebrew developer for the PSP, so I have no idea about limitations and feasibility but I’d love to see the classic Grand Theft Auto series, 3D Realm’s First Person Ninja Assassiner; Shadow Warrior, Rube Goldberg Machine based puzzle game; The Incredible Machine and non-SCUMM based but extremely cool Discworld adventure game series." Any other preferences?