REAPER is a professional Digital Audio Workstation (DAW).
If you're unfamiliar with the jargon, essentially this means that the program is a high-end and very configurable digital recording studio. You can import any audio or MIDI source, maybe plug in a microphone to record something, then loop, pitch shift, trim, time stretch, fade and otherwise edit your work until you're happy.
REAPER is able to map control surfaces (even game controllers) to faders, plug-in controls, REAPER actions and more, helping you to create a working environment that works just the way you expect.
Need more? The program supports all the major audio and MIDI plugin standards: VST, VSTi, DX, DXi, JS, and AU (the latter being OSX only), and so can be extended in just about any direction you like.
And all this comes at a surprisingly reasonable price, for the DAW market at least: if you're happy with the trial version then REAPER can be yours from $60. See the REAPER site for more.
What's new? See the changelog.