Audacity is a free multi-track audio editing program that runs on several platforms including Windows, Linux, and MAC OS. It permits recording from a variety of sources and producing a variety of sound file formats. It has features for pitch and tempo shifting, noise elimination (such as clicks and pops), cropping or extending recordings, adjusting bass/treble balances, and much more.
CALLERLAB recently ran an education session on Audacity and you can find the handout here. The Audacity website contains full documentation under the HELP selection on the masthead.