The Gertboard, by Gert van Loo, is introduced over at the Pi-site blog. It is an extension board for all the tinkerers out there. It provides LEDs, GPIO, sensor inputs and motor drives. The plan is to sell it alongside Raspberry Pi. Personally, I think this is what really will set the Pi apart from other boards. Price might be a factor, but having a powerful, super-Arduino like platform with an open source software stack will change how projects are implemented fundamentally.