When I was a young musician my band director gave me a copy of Jamey Aebersold Vol. 24: Major and Minor, which is a collection of fairly simple backing tracks in various keys. It was a great resource to practice basic improvisation and get better at my scales. Nowadays I suspect that students might be a little less interested in traditional swinging jazz styles, so I've started to collect together rock, electronic and hip-hop tracks that can serve the same purpose.

Take a look at the circle of fifths diagram below, and click on the links for content in each key. We are just getting started collecting things together, but eventually I hope there will be lots of options for all of the keys.