Here are few websites I personally find really usefull for learning to play along with backing tracks (and more):
ActiveMelody is devoted to helping guitar players improve their ability to improvise lead, rhythm, and fills. New lessons are added weekly to both the free and paid levels. The paid level lets you download the mp3 backing track for each lesson which you could upload to Backing Buddy.
Great site for finding free guitar backing tracks for popular songs as well as jam tracks! The backing tracks can be played from the guitarbackingtrack.com website or downloaded in mp3 format and uploaded to Backing Buddy.
A great site with over 1000 completely free guitar lessons!
Inexpensive quality backing tracks for tons of differnt songs. An easy to use interface lets you mix the backing track just as you would like it to sound.