Jammit (free beta software, $1-$5 per song - pricing is in US Dollars) is a great tool for the budding or working musician looking to learn classic cover tunes quickly. See all software reviews.
Every song you load into Jammit is a real recording of a popular song, but instead of having to pick the instrument or vocal out of the mix with just your ear and brain, you can solo it, remove it, turn it down. You can also see the sheet music for the part right in front of you.
Once you've loaded a song in Jammit, you'll see multiple copies. Each is mixed (or de-mixed) especially for one instrument or the vocals. Load the version for the part you're interested in learning and have at it. You can switch between notation and tablature (for guitar and bass parts), and there's drum notation as well.
The mixer is the magic in Jammit as it lets you vary the volume of the band, isolate instruments and vocals, and add in a click track.
The transport controls let you halt playback, slow it down to a pre-determined tempo (90 percent through 50 percent of the original tempo) without changing pitch, and jump back through the song in sections. There's also a record feature so you can hear what your own licks sound like against the original song.
For all its pluses, Jammit could still be better, especially for fast instrumentals.
Slowing tempos beyond 50 percent doesn't sound great, but there are songs with fast passages where it would help. Also, you can't simply load MP3 files to take advantage of the slowdown capabilities; you're limited to the processed tracks available in the Jammit store. These cost from $1 to $5.
If that sounds expensive, keep in mind that each track has royalties due and must be de-mixed and notated--no Little Feat, pun intended. (The library is heavily slanted towards classic rock and/or songs with distinctive licks)
Quicktime Pro is very good for varying audio tempo and looping, and the free VLC does it as well. Also, the Jammit PC help file is a bit weak in the beta I tested, but the interface is intuitive and the program rock solid in terms of performance. Jammit is also available for OS X and iOS.