Guilty Gear
Easily the best and most complete English language Guilty Gear resource on the Internet. It has frame data, a frontpage with news regarding the tournament scene in the US, and a very active forum.  Since 2008 Blazblue has kind of taken over the forum, resulting in much less activity in the GG sections.  It’s still worth reading over some of the old character threads if you’re learning the game, though.

A blog that tracks match videos of various fighting games. Updates daily

Adam’s Myist
The Slayer player Adam who does commentary on many GG vids from Takadanobaba Mikado has this mylist, which contains all of the Mikado GG tournaments.

Mizuumi is a site dedicated to being a resource for fighting games other than Street Fighter IV. A noble task, and it has the best English language GG wiki out there.

Good archive of graphic rips for most of the GG games. Most of the Millia pictures in the Official Art section were taken from this site.

Heaven or Hell
Has all palettes of every character from every GG game starting from X. The Millia palettes are taken from this site.

Japanese Guilty Gear Wiki, very useful resource if you can read the language.

Japanese wiki for custom color palettes for the PC version of Guilty Gear XX #Reload. There are lots of cool downloadable palettes for every character on here, especially Millia.

Pretty much THE group that does videogame OST rips.  All of the .mp3s in the Millia music section were ripped by them.

Youtube Accounts
This Youtube account has an almost complete archival of every Accent Core video released before 2009.

Starting from 2010/10/15, a-cho no longer offered videos for direct download on their site, and began to put all of their ranbat and tournament videos, including GG ones, into this account.

a-cho puts their post-tourney/ranbat casual vids on this account.

Youtube account that has all of the a-cho ranbat and tournament vids before 2010/10/15.

Kansai Order Sol player Hama’s youtube account, has videos of all of the Kansai GG danisen series

Has a lot of GG combo videos

Has a lot of the Mikado GG tournament videos from 2010

Mostly Blazblue, but has a fair amount of GG vids as well.