Maxconsole has some good psp hacking forums. There was another site that has pretty good coverage and downloads, but their layout is shit. QJ something or other? Found it. pspupdates.qj.net.
I've only tried a few emulators on it. The psp port of SCUMM VM is very good and available at the official SCUMM VM homepage. I also have a port of a gba emu (Virtual Boy, I think) that runs gba games at full speed, although it sets your psp to 333mhz to do it. Very nice emulator.
There is a SNES9x version that runs pretty well. More or less full speed with sound, although some spots in games or Mode 7 slow it down. It seemed to be developed by a Japanese guy though and finding info about it or any sort of documentation was very hard. Also confusing the matter was that there were a couple other offshoots of SNES9x that weren't quite as good.
The last emu I tried was a CPS2 one. It worked fine on my old firmware, but when I updated to a newer one, I had a real hard time getting the program to work again. It plays the games great, but setting them up is a pain in the ass. You need to run some program on your pc that takes the rom and generates some cache file from it that the emu needs to load and play. You have to do it for every game you put on there. Convoluted to set up, but once you do, it is a ton of fun playing D&D, AvP, and SSF2T on the go.