Bryan Air Service - "10,000 feet and Climbing - The Aviation Adventures of Richard Schreder", by Karen Barbara -  
ASA - US Exam Preparation books and software
College Park Press - Very interesting Fundamentals of Sailplane Design book
Cumulus Soaring, Inc.
Kai Gertsen - Excellent Tutorials on Cross-County Soaring - Free Download
   - Introduction to Cross-Country Soaring
   - Off-Airport Landings  
GliderBooks.com - Comprehensive Flight Training Manual and Knowledge Handbook for Gliders
JP Aviation
Knauff & Grove Soaring Supplies
Soaring Books and Supplies - Bob Wander
Wings & Wheels