A few of the other posters mentioned LMMS, and I have a little experience with it too. The interface is far from elegant and I don't like the way it manages plug-ins. In fact, if you're on Windows there are plenty of other better alternatives. On Windows at least, the thing that pissed me off about LMMS is how crash-happy it is. Maybe it was b/c of memory allocation settings or something, but it would crash a lot without any apparent reason, usually when I loaded in a vst-i. Most of the vst-is I tried crashed the program, so it was hardly productive.
If you were on Windows or MAC I'd 100% recommend MuLab, however. Been using it for about 8 months. Very elegant and clean interface, and works with every plug'in I've thrown at it. Also has some great bundled samples and instruments, and pretty nice editing features for samples directly in the program (you can crop them, quantize, gain fade, etc and cut new samples directly from a sample already loaded in).