GTA – Official

Rockstar North
Scotland-based (primary) developers of the Grand Theft Auto series.
Rockstar Games
Parent company of the above and various other Rockstar studios.

GTA – Community, Projects and News
Largest GTA-related message board, home to the fan and modding community. Part of GTA Net.
GTA Modding Wiki
All GTA modding related knowledge in one convenient place. (At least that's the plan.)
GTA Garage
Huge GTA mod database, free hosting of your mods.
Myriad Islands
An open community mapping project for San Andreas. Also visit the MI forum.
Planet GTA
News and general information, updated quite often.
Big German GTA community.

GTA – Modders

illspirit dot com
illspirit has been modding the GTA series since the beginning. He's an admin over at GTA Net, as well as the founder and lead map designer and modeler of Myriad Islands. And if that gets boring, there's always Jack.
When Jernej L. (aka Delfi) isn't busy coding tools for GTA, he works on his own game.
REspawn is a talented map modder, coder, modeler and gfx artist.
PatrickW GTA-Modding
Dutch modder Patrick W. does SCM scripting and related coding and research. But he's certainly best known for having brought doom over TakeTwo with his infamous Hot Coffee Mod.
ODIE's Stunt Park
Heiko "ODIE" Feege was one of the most creative and innovative map modders the GTA community had ever seen, may he rest in peace.
Suction Testicle Man's GTA Page
STM is a longstanding modder and vehicle modeler.
Ocram's GTA Files
Map mods, tutorials and scripts by Ocram.
LZF Downloads
LazioFreak's mods, maps and tutorials.
Project Cerbera
Cerbera has been a modder since the first GTA, and wrote various articles about GTA, functional webdesign, and how to avoid washing.

General News, Information and Entertainment

The Register
A British IT news site, not lacking a healthy dose of sarcasm. Home of the BOFH.
Community-powered tech news site for nerds.
Game development industry news and articles.
Game Politics
Frequently updated blog about the politics of video games.
The Daily WTF
A daily updated collection of funny and/or scary anecdotes, code snippets and screenshots from the IT industry.
A webcomic of romance, sarcasm, math, and language. – Best webcomic I've seen so far.
Cyanide and Happiness
Another great daily webcomic, usually NSFW.
Zero Punctuation
Zero Punctuation is The Escapist's lovely game review column, with a new video appearing each Wednesday. It's exactly what it says on the tin, so you might want to watch them twice to grasp all of the heavily compressed goodness.