I've been playing with SoftDrum, it's free and seems to work pretty good so far. It's a VST plugin, MIDI based...
12 pads, each pad plays up to 4 separate samples which are chosen based on velocity, up to 16 voices simultaneously per pad (or 192 total simultaneous voices at most) so you don't have to worry about sounds getting "chopped off" during a drum roll or fill or something. Separate volume controls, pitch correction, you can group pads with each other so their own sounds can't interfere with other sounds in that same group.. so far it seems decent for being totally free.