I realize that you are probably looking for free sites, but an even better and resonably priced alternative is http://www.smartmusic.com/
The cost is $30 a year for a subscription. It's far better than trying to play along with MP3s because the software truly is "smart". You can easily adjust the tempo and it listens to the player and waits for entrances, fermatas, ritards, etc. They have a vast repertoire for all instruments and I highly recommend it. Teachers can even use it to make assignments. Students can record their "performance" for later listening or submission to their teacher for review.