We bought a Roomba! This is a vacuuming robot. You can see it in action here. It leaves the recharging base, vacuums the room, then (hopefully) returns to the base when it has finished. It has a sort of random cleaning pattern but can recognise when it has done the whole room. It trumpets when it is about to start cleaning, plays a mournful melody if there is a problem (e.g. can't find the homebase before the battery runs out) and calls out triumphantly when finished.
For more info, it has its own wikipedia page: http://en.wikipedia.org/wiki/Roomba