PSP homebrew lets you do many things. The most cool thing i think it you can put homebrew games and save them to your PSP memory card, so you dont have to put un the UMD disc to play the game.
PSP homebrew Downloads: Here are some really good sites to download homebrew apps and games
http://psp.brewology.com/?page=downloads&cid=3536
http://psp-news.dcemu.co.uk/psp-applications.shtml