Audio

Homerecording.ORG - Excellent home recording forum, helpful members and quality discussions.
SAE - Sound engineering educational institute. My current prison (place of study)!
MusicThing - A great music blog featuring all the weird, wacky, hilarious and esoteric bits of the music industy you've probably never heard of.
CreateDigitalMusic - A great news/blog/forum site with plenty of information on the latest digital music production themed information.

Visual

DeviantART - Submit your art to DA, comments, massive amount of members, often some great quality work. Also has an excellent sister site from which you can order prints of DA images.

Shops

Threadless - Cheap shirts where you can vote on/buy designs submitted by people from the community.
HotTopic - Rock/Punk/Goth/Pop-Culture clothing and accessories, often has sales where lots of neat stuff plummets to prices even people wrangling with the Australian dollar value can live with, yay!

Just Plain Cool

WikiPedia - No doubt you've already discovered the ultra-handy-dandy-ness of WikiPedia, just just in case you haven't, it's essentially an open source encyclopaedia.
Everything2 - Massive community which sort of resembles the love-child of WikiPedia and a Blog site, much cooler than it sounds, prepare to lose hours of your life to this beast.
SourceForge - The largest open source software library I know of, excellent range of both programs, mirrors and general geeky cool. Before pirating that next piece of must have software, check SourceForge for an open source alternative, you might be pleasantly surprised.