They're usually splitted by plataform.
http://www.dcemu.co.uk/ <- overall homebrew/emulation site
http://www.gbadev.org/ <- nice gba stuff...
http://www.dcemulation.com/ <- dreamcast lives on
http://www.gbaemu.com/ <- mostly handheld stuff, but hey, it's the 30th of the month, so bandwidth is out. Try again in 3-4 days.
I'm pretty sure you're set with thefirst link tho. There's lots of other linkage from there.
Also, there's no commercial roms in any of those sites, afaik. If it's against the rules, mod please delete.