There isn't many sequencers for linux, but here is one Rosegarden . Or atleast i hope its sequencer, because i haven't test it yet . Tho i am pretty sure you already found this program, as it pops out first when searching with google. And as you probably noticed, vsti (virtual studio technology instrument) can emulate instuments, be softsynths and so on. Tho free ones usually aren't very good. Also here is link to pretty decent realdrum wavepack and the best part of it is, that they offer it totally free to do what you want with it.