HexBug Nano (single)
This robot acts like a real bug. It will skitter around, even if it flips over it will right itself and keep on going!
Works great on wood floors, but doesn't work on carpet.
Super fun and very entertaining.
Comes with a battery already installed, and you can replace it later after hours of fun.
Warning: it's small, so please keep it away from small children and pets. No one under 3 years old please.