A Tom Thumb is a fairly popular western bit. It's a short shanked bit with a jointed (or broken) mouthpiece.
It's a very controversial bit - because a lot of people think it is a very mild bit, but it's actually a very harsh one. (Which is a problem - because if you're riding and think your bit is milder than it is, you can annoy or even injure your horse).
Yes. I don't like Tom Thumbs. Occasionally a bit of opinion slips in here. I particularly don't like that they're often sold with cheap western headstalls.