Audacity is a free, open-source audio recording and editing program available for both Windows and Mac OS X. It is extremely simple to use while being powerful enough to record, edit, and present a wide variety of audio files. These instructions are written for the latest version of Audacity (2.0 as of April 2012). The differences between the Windows and Mac OS X versions are minimal.
A 10 minute audio recording .WAV file will be about 100MB. A MP3 file will be about 10MB. iTunes can convert a .WAV file into an .MP3 file as well.
Audacity can edit just about any type of Audio file.