AzBilliards - Great source for information, forums for every facet of the game 

Dr. Dave's Billiards - Ton of free information on pool, the FAQ and video clips sections are very nice.  Also check out the Billiard University stuff, it's great!

Bob Jewett's Site - Long time writer for Billiards Digest, contains article compilations, progressive practice info, lots of other great links

Dead Stroke University - Teaching site for my good friend and excellent instructor David "Blackjack" Sapolis.  Check him out, he's great!!!

Joe Tucker's Site -  Another great instructor, free snippets of his great racking and practice videos.  Also check out his awesome Youtube channel

Barenada's Pool page - Nice site with some good drills and videos

YouTube - Hundreds if not Thousands of free hours of pool matches, instructional information, etc.

PoolDawg - A great site for all of your pool and billiard needs