Unlike a lot of free Digital Audio Workstation software, it gives you multi-track recording abilities and a lot of advanced effects, like being able to correct pitch and remove unwanted noise. There is also a large number of third-party plugins available as well as user-scripted plugins.
The same basic algorithm seems to be used in both tools, but they get at it from slightly different perspectives. Both tools are really flexible and fun to work with.
Runs on almost any platform that SDL supports, and has been successfully built for Windows, Mac OS X, Linux, FreeBSD, OpenBSD, AmigaOS, BeOS, and even the Wii.