The best of a bunch that I've come across is The Amazing Slowdowner, from www.ronimusic.com. You can isolate small parts of the track and repeat them endlessly, change the pitch to suit your instrument, infinitely variable amount of slow-down, and still sounds intelligible when you've slowed it right down. Excellent. There's a free trial version which gives you full features, but you can only use it on the first couple of tracks of a CD.
Cheers
Ray