LMMS
LMMS (which formerly stood for Linux MultiMedia Studio, but now stands for nothing) is a free open source audio composition studio written in C++ which supports input from any MIDI tracker and can utilize soundfonts, VST plugins, and a whole host of other additions. It was first released in 2004 and sees semi-regular updates. Music can be saved in the native MMP format, but can also be exported to MIDI or rendered to WAV, OGG, and MP3. Music can be imported from MIDI or Hydrogen's H2SONG format.
Personal
While looking for a decent free audio composer to test out my new MIDI keyboard, I came across this program. I was initially very impressed by its wide range of features, but I found the interface to be difficult to understand and eventually abandoned work with it. Still, it's the best free audio studio I've seen.
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