Powering a breadboard with USB
USB has the concept of a unit load, which seems to be the cause of some confusion. In USB 2.0 a unit load is 100mA, but a device may draw up to 5 unit loads from a port. In USB 3.0 a unit load is 150mA and you can draw up to 6 unit loads, or 900mA.  

So I don't agree with sybreon when he says that a USB powered device isn't capable of doing much. He mentions LEDs as drawing much current, leaving little for the rest of the circuit. Even if your circuit has ten 20mA LEDs your power budget still leaves 300mA for the rest.