Best Online Tennis Pros:

For Advanced Players (U.S. NTRP rating of 4.0 or better):
Jeff Salzenstein (https://www.youtube.com/user/TennisCoachTV). Former top-50 ranked pro with awesome tips. A must watch site for all aspiring pros.

Heath Waters (https://www.youtube.com/user/theacesystem) and (http://www.virtualtennisacademy.com/).
His Virtual Tennis Academy has some of the best videos for advanced players. Signup is free for now. Hurry before he starts charging for the site.

Peter McCraw (https://www.youtube.com/results?search_query=peter+mccraw+tennis) and (http://www.mccrawmethod.com/).
He is a well-known junior development coach. His "Technical Fundamental DVD" in our opinion is well worth the money.

Patrick Dougherty, the serve doctor at IMG Academy Bollettieri Tennis (https://www.youtube.com/user/servedoc).

David Bailey for footwork (https://www.youtube.com/user/baileymethod).

Hank Pfister for serve and volley lessons (search for Hank P􀀁ster in the search box at http://www.tennisresources.com).

Dr. Calvin Nii (https://tennisspeedresearch.blogspot.com/). Advanced discussion on forehand biomechanics ideally suited for junior development coaches.

For Intermediate Players:
Mauro Marcos (https://www.youtube.com/user/CoachKyril) and (http://mauromarcostennis.com).
He is incredible in explaining technical concepts in easy to understand terms.

Jim McLennan (https://www.youtube.com/user/JPMcLennan).

Tomaz Mencinger at Feel Tennis (https://www.youtube.com/user/TennisMindGame).

Ian Westermann (https://www.youtube.com/user/EssentialTennis).

Florian Meier (https://www.youtube.com/user/OnlineTennisLessons).

Tom Avery (https://www.youtube.com/user/BestTenniscom).

Brent Abel (https://www.youtube.com/user/webtennis).

Will Hamilton (https://www.youtube.com/user/FYB2007).