http://www.vertjump.com/ -Includes free articles to help readers increase their vertical jumps.
http://freeverticaljumptraining.com/ -Alot of tips, exercises, theory etc. All You need to increase Your vertical leap
http://verticaljumpzone.com/ – Very good informative site. Videos, theory.
http://www.verticaljumping.com/ – One of the best site available right now. Very big collection of exercises