Here are some website for backing tracks. They work great for practicing.
Enjoy!

http://www.myguitarsolo.com/

http://www.jamtracksmania.com/

http://nermark.articulateimages.com/

http://www.jamcenter.com/ <-- does not work with some browsers

http://www.chordie.com/

http://www.guitarbackingtrack.com/

http://www.guitarvoice.com/BackingTracks/

http://sourcetabs.com/guitar-backing-tracks/

http://www.guitar-wav.com/

http://faststrings.com/guitar-backing-tracks.html

http://www.guitar-tube.com/backingtracks

I'm sure there are more out there, so feel free to add any that I've missed....