Qtractor is an Audio/MIDI multi-track sequencer application written in C++ around the Qt4 toolkit. The target platform will be Linux, where the Jack Audio Connection Kit (JACK) for audio, and the Advanced Linux Sound Architecture (ALSA) for MIDI,
I used GNU / Linux to make music since around the time Jack was introduced. I spent coutless nights teaching myself how to use just about every single system available, and I always came back to Qtractor. Everything makes sense in Qtractor. And when it seems that it doesn't, a quick message on Rui Nuno's blog and you're out of the hole.
The development is steady, focusing only on the core of the matter, and the result is a solid system, usable right now.
Qtractor does not even try to be everything, it wants to be useful in the middle of the huge Linux audio production ecosystem, talking nice to everybody, implementing new techs & protocols silently, never breaking what's worked so far, bringing it all together. For me, it's the standard against witch I can compare everything else.