While the Raspberry Pi is an inexpensive little computer, all the little accessories can add up. For many applications you'll need a powered USB hub to expand the number of devices that can be connected to the Pi. But you may also be able to power the Pi from the same hub, saving you from purchasing a USB charger. It tried this with the 7 port hub available from Farnell (SKU 2115058, €10) and it worked just fine with a Model A Pi. This compares favorably with a dedicated USB charger which typically costs €6 - €10.
Update (14 June 2013): It seems it's not even necessary to use the micro USB cable (at least in the case of this hub).
Update (19 June 2013): I just noticed that the possibility of powering a Pi from a USB hub that back feeds power was mentioned in the release notes for the Revision 2 of the Rapberry Pi hardware. This is made possible by the removal of the USB port poly fuses that were present in Revision 1 hardware. However it does note that the hub must not supply more than 2.5A under fault conditions (presumably because there is no over current protection supplying power this way).