OK, here's a great little tool: The Practical Meter from Power Practical. This gadget tests USB ports' power output and reports how much power is available. Why is this useful? Because as the company notes: "... not all USB ports are created equal ..."
The Practical Meter consists of a USB dongle and a companion cable that splits into three providing an Apple 30-pin connector, a mini USB connector, and a micro SUB connector. This is also a "fast charge" cable (these are wired differently from regular data cables allowing for a higher current).
Plug the dongle into a USB port, connect it to a device to be charged, and a display on the side of the dongle shows you the power available in a range from 1 Watt up to 10 Watts in one Watt increments (in case you're wondering how that's shown with just five bars the leftmost bar alternates red and blue when the power if above 5 Watts). A great use of the device is with portable solar panels to determine the position that provides maximum power output.