It *was* in this topic? Weird. Could've sworn I read through every post. Clearly not. Thanks for digging it up.
The problem with that (and the reason for asking) is that I've never had the first idea what programs are worth using even to start with. Yeah, one could test their look with a random program from a Google search, but finding a program doesn't necessarily mean you'll find one that's worth fucking around with; if you have no experience with something, you're not going to have the first clue what is and is not necessary for adequate results.
It can be difficult to find one that's featured and not buggy, especially if you're looking for open-source stuff. There's quite a few projects, but most of them seem to be version 0.x. Despite asking this myself a while back, I never got too heavy into the music composition stuff.
Besides TuxGuitar, another one that I've used a bit is Aria Maestosa. Another one that might be worth looking into is Sekaiju, which seems pretty feature-heavy. That's a good thing and a bad thing, because it might be kind of overwhelming and confusing with being able to directly control midi events. That's partly why I haven't really tried it out. Another one that looks fairly capable is Frinika, which can do more than midi but is at version 0.7.1. I also specifically remember using a Java program a long time ago, but I'll be damned if I can remember what is was called, and google has failed me so far... EDIT: it was MidiSwing, and it hasn't been updated for around 2 years, and it's only at version 0.3.6b. Would not recommend it then.