Algodoo

A laser that records the weight, not density, of the objects it hits.

When the laser hits something, the object's collideset turns to 0 so it won't keep adding that object's weight to the record, and so it'll penetrate all the way through your contraption you want to weigh, instead of just weighing the stuff on the outside.

Toggled On/Off with "1" key.

Unfortunately, this scale only works on boxes, so it might not weigh everything in your contraption. Hopefully I'll find a way to weigh circles and polygons, but it might take a while.

Edit: In order to read the weight record, you have to open the laser's script menu and look at _weight. I tried to make a color value display for it, but it didn't quite work.