To the best of my knowledge, all normal VST plugins are compatible with Samplitude and I would presume Music Maker though that is sort of entry level software so I'm not really very familiar with it.
For free plugins there a couple of decent options. Start with Cockos ReaPlugs. There are a set available for use without Reaper.
REAPER | ReaPlugs
Also check out some old VST plugs that are still useful from GVST.
And finally, we have a fellow contributor here that has done some nice work at his business Platinum Ears.
VST plug-ins - essential tools for every recording studio