#1. icoach- The ITF's site with the best content from top international coaches from around the world. It's a paid site, but like most paid sites, for the price of a private lesson you get a lot. This site focuses on all aspects of playing & coaching, not just videos of a guy standing there swinging a racquet talking about technique.
#2. TennisOne - Great content, good articles from international coaches and lots of Quicktime clips of the pros.
#3. Tennisplayer.net - Created by John Yandell, Similar to TennisOne, good articles and lots of clips of the pros.
#4. Ace Coach - Lots of great content about coaching and playing tennis from a 21st Century approach. Created by Wayne Elderton who writes articles for the ITF & TennisOne among others.
#5. Tennis Canada's PTA - A great resource for learning cutting edge coaching techniques. Lots of instructional video and interviews with top coaches.
Sites you may want to skip:
Anything with Jimmy Connors & Tracy Austin called: Tennis Fundamentals.