rbpitch is a plugin for Rhythmbox that lets you change the pitch, tempo, and speed in real-time while playing a song. It needs testing, popularity, and bug fixing -- you can help! Just trying it and reporting your findings here will be immensely helpful!
And you might find it useful, too: I've heard that it is useful for dance lessons, guitar practice, and singing. I personally just enjoy the aesthetics of changing the pitch, tempo, and speed of my music.
The new homepage for rbpitch is on Launchpad: http://launchpad.net/rbpitch You can find downloads, information, source code and a bug tracker there. Binaries are available for Ubuntu 10.10, 11.04, and 11.10! Visit the Launchpad site and look for "END USER INSTALLATION INSTRUCTIONS"! For those running other distros, you can check out the source code and read the file README.txt for instructions to compile from source. This thread is very old and contains obsolete / incorrect build instructions; please disregard them!
My plan for the future is to support the latest stable version of Ubuntu with binary packages that can be installed through Ubuntu Software Center (download a .deb file and double-click it). For other distros running a recent version of Rhythmbox (I currently require Rhythmbox v0.13.0 or later), you will have to build from source. But it is much easier now than it used to be!
rbpitch was inspired by Olli Parviainen's excellent PaceMaker plugin for WinAmp. WinAmp and PaceMaker won't run on GNU/Linux, though. Rhythmbox is an excellent media player, and rbpitch makes it that much better!