Better than anyone's thumbs... this produces a really amazing Thump.

I saw Stig's version of this at the last Instructables Show & Tell. I was enraptured. I had to have one, and of course that meant I had to make one.

Materials:

a power jigsaw. cordless is ideal; bladeless is fine; quiet motor is best. I spent about two weeks not finding anything like this in thrift stores or on eBay and finally bought a new, cheap-ass jigsaw off Amazon for about $30.
one of those old keys you have lying around that you can't remember what it's for. brass is better than aluminum.
a rubber foot for the bottom of a chair leg. 1 inch diameter is a good size
Bondo or similar hard-curing filler

Tools:

Dremel with grinding tip (or metal file, if it's all you've got)
popsicle stick or plastic spoon or some such to mix the bondo
screwdriver of whatever sort your jigsaw comes apart with (mine took a hex wrench)

This is really a very easy project. All you have to do is remove the parts of the jigsaw that have to do with actually sawing, and create a thumping attachment that fits in the chuck to replace the blade.

Step 1: Prep the Jigsaw

Remove the base plate from the jigsaw using whatever means are appropriate to the jigsaw you've got. Mine, as noted, needed a hex wrench.

It is not strictly necessary to remove any plastic as I did from around the chuck assembly, but I found it difficult to get the hex wrench in to loosen the chuck. (Chuck may be the wrong term but I mean the clamp that holds the blade.)

Step 2: Make the Pounder Foot

This contraption is the part that hits you in the shoulder so wonderfully. There are any number of ways you could do this but here's what I did.

Take the key and see how close it is to fitting into the chuck of the jigsaw. It'll be a lot thicker than a blade so most likely you'll need to grind it down. Note that the pictures show two different keys as I made two of these. Note also that aluminum keys are not as strong as brass keys, and if someone tries to massage their lower back while sitting in a chair and leaning back on the jigsaw at an angle, the key may break off.

Once the key fits in the chuck, attach the chair leg tip to it (you may need to grind down some of the key head too). I filled the tip with some Bondo to support it, give it more heft, and attach it to the key. Mix a small amount (a tablespoon or so if plenty) according to Package Directions and squish it into the chair leg tip. Push the key head into the tip, you probably have to squeeze the tip to make it wide enough. Let the bondo cure, which will take between 5 and 15 minutes depending on the mix proportions.

Step 3: Fix the Pounding Foot Into the Chuck and Massage Away

Insert the key into the chuck and tighten it down well (you don't want it flying off during use).

It's easier to use on someone else but possible to massage yourself. As demonstrated, it's good practice to start the jigsaw engine before approaching the muscle with the pounding foot.