You can organize playlists by BPM and then practice in a particular BPM range to your music without getting bored listening to the same song over and over and over and over...
The Amazing Slow Downer allows you to adjust the speed of an audio file so you can transcribe or practice along to an audio file at any speed you want. The pitch of the file does not change while you vary the speed. You can also export the audio file to be whatever tempo you wanted
This website allows you to upload sounds onto a profile and embed those sounds/playlists into various web applications. There is also the ability to comment directly on the wave form which allows for specific feedback.