Coursera offers online courses taught from over 60 universities for free. I recently learned of one offered by Berlkee College of Music an Introduction to Jazz Improvisation, taught by one of the greatest improvisers ever, vibraphonist Gary Burton. All you need to take this course is a basic understanding of chord symbols and improvising over basic progressions (blues, etc.), have an intermediate ability on your instrument, and have the ability to record yourself and convert the audio to MP3 format.
The next 5 week session starts up next Monday (April 29, 2013). If you want to participate in this one you’ll need to sign up soon.