ASA - US Exam Preperation books and software
AviationWise1- Aviation Ground School and Test Preps
Basic Sailplane Instruments - Nice description of the basics of variometers by Mike Borgelt.
Bronze Badge Study Guide - Soaring Safety Foundation - U.S.A.
CarrotWorks.com - Very nice presentations on: Lift, Drag and Glide Performance, Control and Stability, and Glider Instruments
Cumulus Soaring, Inc. - Link to books related to Test and Checkride Preparation 
GliderBooks.com - Comprehensive Flight Training Manual and Knowledge Handbook for Gliders
Glider Pilot Ground School - Colorado, training and training books
Glider Pilot and Flight Instructor Practical Test Guide - Jim D. Burch
How to Fly Gliders and Pass FAA Tests - CD by Jim D. Burch
Martin Kroke - Familiarization / training / safety / practice for towed take off, landings, slope-, thermal- and wave flights, distance mountain glider flights, aerobatic flights 
Master Instructor Cross-Country Program - Soaring Safety Foundation - U.S.A.
Soaring Books and Supplies - Bob Wander
Soaring Safety Foundation - SSA - On-Line Learning Page, Tow Pilot Course, Wing Runner Course
Tow Pilot Course - Soaring Safety Foundation - USA
Wing Runner Course - Soaring Safety Foundation - USA

Links no longer active
Arizona Glider Stuff - Jim D. Burch
Cumulus Courseware - Multimedia Computer Based Training
Practical Test Standards (PTS) (U.S.A.)
Top Ten Ways to Fail a Practical Test (and how to pass one) - USA - Jim D. Burch 
WebExams.com - Free Online FAA Practice Exams