I've heard of some meditators who use tools like EEGs. Of late these models are becoming pretty cheap, USD 100-500, which was unthinkable even a few years ago. They can't be very accurate at this price but they are there, see for example Neurosky Brainwave.
At least in theory they can reveal whether thoughts stray frequently or stay in one spot, and we can train using the feedback to move brain waves to certain areas and stay there. I don't immediately see how this will add any more than novelty value to my meditation practice, since I already know the familiar zones my brain hits by observing the breath and other bodily signs. If I was more curious I'd probably read up on the possible patterns of breaths and their associated brain states, as far as I know the advanced Buddhist meditation literature is pretty encyclopaedic about such patterns.
Of course, such tools might be very useful - having never tried one I can't be certain.
A lot of reviews I've read claim these don't work very well. They seem to be mostly marketed for kids with ADD, but the geeky meditator is an emerging sub-category.
If someone I knew had one around I'd try it since I am curious, but since I know it'll probably be a useless toy on the shelf after a few weeks, in the spirit of letting go, I've skipped buying one for myself.