if comes to tracker on pc then its reason.
reason has vst support too since ages, got excellent sound quality, amiga mode, a wonderful gui - its the best tracker of all time (got mac osx version, even ppc, i've played with it on my peg with the help of mol). i can even use it to boost my old digibooster modules (save them in xm with digi).
its not freeware duh but cheap and worth the money 100%.
a must have for tracker users.