OK, this goes back probably close to 10 years ago, so the details are going to be sketchy.
I remember visiting the BMOS and seeing a machine that had supposedly been running for (at the time) over a year, with no power supply in evidence. All I remember about it was that it was a continuously spinning wheel on some kind of mount, with some tubes pointing at the rim of the wheel at an angle. Sorry I can’t be more clear. I also seem to recall that there was a contest to guess how it worked.
Did anyone ever see this exhibit, and if so, do you know how it worked?
BTW, I was reminded of this by a rerun (from several years ago) of an episode of Star Trek Voyager (“Future’s End”) that was on recently; the character played by Ed Begley, Jr. had something that looked very similar in the background of his office (I didn’t watch the whole thing, so I don’t recall if it played a part in the episode).
This doesn’t ring a bell. I’ve been back in Boston for over ten years, visiting the Museum all that time, and I can’t recall a Perpetual Motion exhibit. There IS a kinetic sculpture in the lobby outside the Mugar Theater and Hayden Planetarium that has a wheel like the one you describe, but no one claims that it’s perpetual motion. (I think it’s called “Archimedean Cogitation”, and it’s by the same guy who built the kinetic sculpture in one of the terminals at Boston’s Logan Airport.) In fact, the motor that runs the chain conveying the balls up to the top is clearly visible.
There ARE perpetual motion machines similar to the one you describe. You can see pictures of them and others in David Ord-Hume’s wonderful book, Perpetual Motion. There is a device described and pictured in that book that has been operating in a London museum for over 175 years (with brief periods where it didn’t work because the humidity was too high). That device ran off a home-made battery, and is to my knowledge still running. It would make for a heck of a battery commercial, because it literally keeps going and going. It was discussed on the SDMB several months ago.
But I don’t recall anything like this at the Boston Museum of Science. You might try writing them at http://www.mos.org .
Thinking back, it may have been as much as 15 years ago.
The mind’s the first thing to go, right? 
I’ve been taking the kiddies to the Museum of Science for quite some years now. I remember seeing the device there, but not seeing an official explanation. As I recall, it was on a tour of various science museums. It wasn’t claimed to be a perpetual motion machine; it was a contest for the best explanation of how it worked. My guess is that the 5 little plastic boxes on the rim of the wire-spoked wheel each contained a small AAA-battery-powered electromagnet, and that the 4 sets of plates they passed through, mounted to the frame, were permanent magnets. With that arrangement, there would always be a net torque to turn the wheel. The rod and disk thingies at the bottom were red herrings. It would be simple to change batteries after the museum closed.
The sculptor Cal is thinking of is George Rhoads. There are 2 smaller but similar kinetic sculptures of his in Terminal C at Logan, but they’ve unfortunately fallen into disrepair.
I remember reading an article about a (non) perpetual motion machine. I believe the machine and the article were by Martin Gardner. He said something like “It’s not a perpetual motion machine, but it’s powered in a way that no sane person would ever think of.” And he related the story of some folks trying to guess how it worked, saying “It probably just has some magnets hidden in it,” which made him terribly disappointed that people could confuse a magnet with a power source.
I’ve always wondered what it looked like and how it actually worked. Hmm. It’s probably the same one, yah?