Audacity is a free, open-source audio-editing tool. It can also be an invaluable tool for guitar study.
I use the Beta 1.3.7 version. Under the Effects menu, you can use the Change Tempo option to slow down an MP3 without affecting its pitch. I usually will reduce a file's speed to 1/2 that of normal which is a setting of -0.50.
This allows me to hear the details of a fast passage and compare my accuracy and timing at a slow speed.
