EmuParadise.me - A giant collection of emulators and roms. One of the best!

CoolRom - A Rom collection with only working dumps. Great for quickly finding known roms.

ROM Collector - A cool site where you can purchase ROM sets on CD for a nominal fee, mailed directly to you. Very cool for the avid collectors out there.

GameBoy Advance ROMs - This site specializes in GBA ROMs. They have anything and everything related to GameBoy Advance.