"Vanilla" (basic) changes for many jazz standards. They also have wonderful back up tracks to practice with.
Excellent jazz blog. There is much improvisation information here, especially for advanced techniques.
Excellent resource for jazz piano. The interface is a little awkward, but it is functional and has great information. you will have to make a log in to access the lesson material.
stephanewrembel.com/ - Gypsy Jazz Back up Tracks
Fun back up tracks, but you have to guess the key.
This is meant to be an alternative to automatic jazz backup software like band in a box, and while it is not near as feature packed, it is functional, and free. You'll have to join a yahoo group to access pre-made song files.