Midi files are abundant on the web, and many of them are helpful for learning songs. Some even provide full accompaniments. Here are some of the sources out there:
Kunst der Fuge has a very large collection of classical repertoire in midi format.
Hamienet has a wide variety of midi files, but their strong point is music from Broadway musicals. This is especially helpful for learning Broadway showtunes, or preparing for an audition, because many of these midi files are full orchestrations of the song. In other words, an accompaniment to practice with. A little heavy on the pop-up ads.
Free classical sheet music and midi files.
While the Choral Public Domain Library is primarily a repository for choral scores, it frequently provides MIDI files of the same, so can be quite useful for learning music.