The strumstick is a handmade instrument designed by Bob McNally. Most models are diatonic (like the Appalachian Dulcimer), but mine is the "Long-Scale 3-String Chromatic." My dear wife gave this gizmo to me for my back in 2004 and I've been plunking happily on it since. I usually fingerpick it rather than strum it though, so maybe mine should be called a pickstick.
There are two tunes on my album Dulcimers Are Go that utilize the strumstick. Here's one of them: