Audacity is a popular sound recorder and audio editor. It is a capable program while still being easy to use. The majority of users are on Windows but the same Audacity source code compiles to run on Linux and Mac too.
Audacity is a free cross platform multi track audio editing program from Sourceforge.net. It will let you record, edit, and mix an unlimited number of tracks. Audacity runs on Windows (98 through XP), Mac OS X, and Linux.