(NOTE: Links are in no particular order)

www.romhacking.org
all about ROM hacking. lots of tools and documents. but sadly, no longer hosting sites...

Confederated Translation Company
way-cool group of game translators/hackers

Magick Ink.
lotsa stuff about comics n' cartoons, also home of "Magick Ink. ROM Hacks"

Acmlm's ROM Hack Domain
is it just me, or is it strange in here? (home of the excellent "SMB2 Discombobulator!")

more cool hacks than you can shake a carrot at

EarthDrake Software
spectacular ROM hacks, a message board, and Elric! what more do you need?

Zophar's Domain
emulation news, emulators, utilities, ROM hacks, links, etc.

The Sardius Experience
rarer-than-rare ROMs, game manuals, an undumped games list, a message board, and one cool mofo named Sardius

TheRedEye
ultra-rare, freshly-dumped ROMs, each with its very own detailed review section. a must-see!

TEKHacks
original ROM hacks and ROM hack reviews/previews

The Pit of Shite
ROM hacks n' stuff