Here are the best emulators I've played, in order of how far along they are:
Pretty Much Perfect:
GP Engine, a Turbografx16/PC Engine emulator. Full speed with sound at a very low frameskip. Just about all it doesn't have is save states.
SMS32, a Sega Master System/Game Gear emulator. Not much to say about it; it's basically everything an SMS emu needs to be.
FMSX32, an MSX emulator. Don't know much about the MSX, but this plays Vampire Killer and Metal Gear 2 just fine.
Needs Just A Tiny Bit Of Work:
NesterGP, an NES emulator. Full speed with sound at like 3 frameskip, and full SRAM support. Next version will be even faster and probably have fixed save states and Game Genie support.
GP SCUMM: Not exactly an emulator, but close enough. A SCUMM interpreter for those great old Lucasarts point and click adventures like Sam & Max and Day Of The Tentacle. Only thing wrong with it is that it hangs after certain cutscenes, and after the Whack-A-Rat game in Sam and Max, making that game unfinishable.
I hope it's still being worked on, because except for that bug, it's perfect...
Freeware Games:
GPDoom:
A direct port of the Doom engine to the GP32. You need to supply your own WADs though. Plays great. It's a bit dark, so hope you have the frontlit GP32.
No cheats yet, but ehh.
Slyro The Radioactive Cat:
I don't know why, but I love this game. There are fruits, and you have to shoot them down diagonally or they'll knock out bits of the platform you're standing on. If you've played the mini-game Pyoro from the GBA game Wario Ware, it's basically that, although Slyro has a lot better graphics and sound.
Drunken Frogger:
Remember Frogger? This is that, but...different