Pseudonym beginners tutorial.
DXArts: From Beginner to Winner in only two weeks!
Mutant Sounds beginners tutorial
Another nice intro: workshop material from RedFrik.
Dewdrop world tutorial for intermediate users.
Nick Collins' Computational Music course using SC.
SuperCollider Users mailing list: official conduit appears to be down, but participation is possible via Resynthesize or Nabble.
Any more? Add them in the comments.
(updated on 2nd March 2008, changed title to something more specific, and added links left in comments.)