I've messed with Sunvox; great program, lots of fun, and it was free last time I checked,
I also have Renoise which, in my opinion, is the king of them all.
You can get a free demo and try it.
But, apart from experimenting, I never really used either as a compositional tool; although, it's on my bucket list to actually dig in and learn the program and try to apply it to my regular workflow. (Renoise also makes a program called Redux which basically runs in any daw as a plugin).
Try the demo! There are spotty videos here and there on YouTube; but really, like any other program, you just need to get under the hood and mess around.
Let me know how you like it!
Tom