Audacity is written using the multiplatform wxWindows C++ API, and today runs on most Unices, including Linux, and Windows and MacOS 9/X. Audacity is licensed under the GPL.
You can import Ogg Vorbis, WAV, Mp3, Midi and other sound formats and then select to do your manipulation with the ease of cut, copy and paste (and with unlimited Undo), you can mix tracks together, or apply effects to your recordings. The great thing about Audacity is that there are no limits on the number of tracks or the length of any track, except the size of... your hard disk.
The application has a built-in amplitude envelope editor, a customizable spectrogram mode and a frequency analysis window for audio analysis applications. There are some built-in effects like Amplify, Echo, Fade In/Out, FFT filter, Reverse, Phaser, Bass Boost, Wahwah, Noise Removal and others.
Audacity not only includes many high-quality effects built-in, but also lets you use plug-in effects in the VST format. There are many free, shareware, and commercial VST plug-ins online that do everything from Reverb to Noise Reduction (these need to be specifically recompiled for Linux or MacOS though).
Audacity is a non-destructive editor, but it also writes changes made to the audio to disk, eliminating the need for complicated real-time processing.
There are also "Label" tracks allow you to annotate waveforms (for example, transcribing speech) and later export the waveforms to a text file.
Audacity supports some spectral features that allow the user to view waveforms as spectrograms or plot the power spectrum of any region of audio, and even export this data to a spreadsheet.
Audacity biggest plus is its ease of use. The user interface is extremely simple, however, there are not ToolTips and when resizing its window it flickers really badly (at least under Windows).
Other problems we encountered is the fact that Audacity feels a bit slow when converting an mp3 to raw or wav, while when playing a converted mp3 with Audacity the quality is serverly worse than the original. I read that version 1.2 will support 32 bit sound quality though.
Some Windows users may miss the fact that WMA is not supported, while there is no embedded mp3 encoder (Audacity uses the third party Lame encoder) for copyright reasons.
I feel though that the main features that are missing is something like a "Delete Silence in selected region" option, while there is no Loop mode or Loop-points.
Audacity is perfect for amateurs or professionals who want to do things fast, with a 2 or 3 step procedure. Audacity is not going to take Reason's place anytime soon, but it is a free multitrack editor, which are not so easy to find anyway. Give it a try.
Overall: 8 / 10