Learn to Play Guitar! Justin, of JustinGuitar.com (not the dork with the TV camera strapped to his head) offers over 100 free video guitar lessons for absolute noobs & guitar enthusiasts, with lesson categories & written instrx as well. For people who want tablature to play along, the good people of Guitar Video Tabs provide full tab notation under videos hundreds of popular songs.
While some people like their Kottkes all modern & full of links, I'll take mine old skool. Ladies & gentlemen, the greatest 12 string slide guitarist that ever lived.
The Riff-O-Matic will help you learn to play rock & roll guitar, or at the very least, will help you play several of the most famous riffs in rock & roll history. Using a combination of sheet music, tablature notation & embedded (flash) audio & (windowsmedia) video, the site will get you up & playing the intro to Stairway to Heaven in the guitar store in no time. If you don't have time to learn whole songs, there's even an abridged list of the 10 Greatest Rock Riffs of All Time.