Check out this amazing minature 120 watts power supply (called the picoPSU) which is being used for mini-itx motherboards. It's about the same size as a standard atx plug:
picoPSU review
pics of picoPSU in action on a mini-itx motherboard
Another review: "Testing revealed that the "world's tiniest 12V DC-DC ATX power supply unit" is no weakling, and that it can provide ample power to keep a Mini-ITX system running reliably."
picoPSU review
pics of picoPSU in action on a mini-itx motherboard
Another review: "Testing revealed that the "world's tiniest 12V DC-DC ATX power supply unit" is no weakling, and that it can provide ample power to keep a Mini-ITX system running reliably."