Audacity is an user-friendly, free & open source audio editor. It is currently available for almost all operating systems, including Windows, Mac OS X, GNU/Linux and others. The software gets great reviews and ranks up there with some of the best in it’s genre because of it’s “clean interface, excellent features, and support for 32-bit floating-point audio” (and a great price if I may add). With Audacity, you can easily record/import audio, edit the files, add effects, and export to your desired file format.

Audacity has quite a few build-in sound effects already, but if that’s not enough for you there are multiple plug-ins already available to expand the capability for this software. Some of the things I found very useful:
– unlimited undo & redo (love it!)
– the ability to mix tracks with different sample rates or formats (and have Audacity convert them automatically in real time)
– the Plot Spectrum command (to analyze the frequency)
– the Beat Analyzer (which helps you determine the beats per minute).
Even if you are not planning to become a DJ any time soon, this app may prove useful to plug your voice over a song, cut your own cellphone ringtones or even convert your old audio tapes to digital formats. The interface is not overly pretty, but definitely very clean and useful.

Related Links:
Audacity’s Website
Audacity on Wikipedia