Ocnce you have downgraded your PSP to 1.5 and the upgraded it to the custome 3.03 OE-C firmware it opens your PSP up to use for a lot of things.
First you can run PSP emulators. There are many including the PSPUAE Amiga emulator. You just copy the emulators to the memory stick in the PSP following the instructions that come with each emulator, add some game roms and then you can launch the emulators.
You can do the same with the homebrew games and programs available.
Next you can run PSP game ISOs. There are full ISO images of PSP UMD game discs. You just copy them to the PSP memory stick in the ISO directory at the root of the memory stick and then you can boot the games diirectly.
And finally PSX games. You can also convert Playstation One games and then run them on the PSP just like a PSP game.
A good site to start with is http://psp-news.dcemu.co.uk/ as it contains links to most available emulators, homebrew games and lots more.
Some other cool sites worth looking at include:
http://www.pspuae.com/ (Amiga emulator of course)
http://yoyofr92.free.fr/psp/snespsp.html (the great PSP SNES emulator that is perfect)
http://www.pspbrew.com/ (another general scene site with a lot of information and links)
http://www.psp-hacks.com/ (contains most files needed for anything PSP)
http://forums.maxconsole.net/ (this forum contains the most information and knowledgeable people in the PSP scene. This forum is the first place most things get announced and discussed and this is also where hackers such as Dark_AleX release their custom firmwares. It also has a lot for other systems too).