Just came across this piece of software (yes Mac and PC).

For all (us) bass or guitar fans, finally take an mp3 or a wav song and place it slower without changing the key! Really amazing and does the trick.

Take a look here at http://www.ronimusic.com/

Must've taken ages to come up with the name for it!!!

you can also do it in windows media player - play a song from your library, click on the top "view" menu, select "enhancements" then "play speed settings", You will see a slider which you can move to adjust the speed, without changing the pitch.
Give it a try!

Great tools for learning songs!!
you could also try Riffster, who do a similar program that's really easy to use:D
you can slow a song down or change the pitch which is great if you play de-tuned.
Keep rockin'
B)

Indeed very cool as well !
http://www.novadsp.com/riffster.htm

Just ordered a tascam Basstrainer... that way you can do it anywhere (well, the reason it sold to me was the fact that it have tuner and metronome built in, and that i also can use it as a CD player if i get some good speakers.)