The Raspberry Pi has a micro-USB jack for power input. This can be used with any recent mobile phone adapter. If you use a two-part adapter, with a plug-in AC-DC converter and a USB A to micro-USB A cable, it's easy to measure the current drawn by the Pi.
To do this, you'll need a USB A male to USB A female extension cord and an ammeter or multimeter with a 1A or 10A range.
1. Remove the outer insulation in the middle of the USB extension cable. Peel back the shielding (silver braid and/or foil) to one side.
2. Cut the 5V supply wire (usually coloured red).
3. Connect your ammeter or multimeter to the cut 5V line.
4. Insert this cable between your AC-DC converter and the USB cable going to your Raspberry Pi.
So, how much current does the Raspberry Pi draw?
It looks like the Pi can draw anywhere from 250 to 500 mA in normal operation, though I did see smaller values in the early stages of startup. When idle, my Pi draws 320-380 mA; with a basic Logitech keyboard and mouse attached and in use, and with the CPU and GPU fairly active, it comes close to 500 mA.
Update: Powering the Pi from a Laptop
The fact that the Pi's current consumption is reliably under 500 mA means that it is actually safe to power from the USB port of another system. This is convenient for developers on the go: for example, I'm in an air-conditioned library escaping the current Toronto heatwave, and have my Pi connected to the back of my laptop with a micro-USB cable for power and a crossover ethernet cable for data.