Dj Sheepwolf Mixer 4 This thing is pretty cool. Particularly that it's done in flash. Sounds good for a music "toy". I had fun at least.
Only thing it needs is a programmable sequence bank for the keyboards. You know, for doing arpeggios or some kind of simple user programed melody loop. Putting stuff in and then pushing stuff through an array on a limited beat timing cue seems like it should be possible. It would make up for the awkwardness of trying to play music on the QWERTY layout.
Yet I wouldn't go as far to ask for any filters though, knowing how flash is, that's really pushing it.
As for those really wanting a lot more than what this gives, I'd recommend actual stand-alone software. LMMS as the FOSS option or FL as the commercial pay-for option. Whole other learning curve though, since those don't pre-package everything to synch up with levels and stuff nicely pre-mixed. (Yet they're quite fun and surprising to work with if you keep at it.)