The ones for mosquitos, or is it mosquitoes? Where's Dan Quayle when you need him? Anyway, the mosquito repeller works differently than the ultrasonic pest repellers. The mosquito repeller usually comes in two forms. One that mimics the sound of a male mosquito, and one that mimics the sound of a dragon fly. The claim is that the female mosquito will try to avoid the male, and that the mosquito will try to avoid their natural enemy, the dragonfly.
How effective it is, I couldn't say. From what I've heard, opinions are split.