Our guitar buddy Elmo Karjalainen has recently created a new site, musicbackingtracks.net where you can download backing tracks for free.
As of this writing he is offering two tracks in the style of Jimi Hendrix and Steve Vai, as well as a track off his Unintelligent Designs album with more to be available soon. The backing tracks come with and without lead guitar so you get a sample of how Elmo plays over them before venturing out with a solo of your own.
To get you started, also included are some handy tips and ideas for each backing track which you can use to incorporate into your solos.
Don't forget to check out my interview with Elmo Karjalainen.