I found a way to make a power supply for the arduino with a 9v battery and a power adapter on the arduino playground site, but I thought that the Arduino only uses 5v. Will the 9v power supply harm the arduino?
The power connector as well as the Vin pin are connected to a 5V regulator. This regulator takes voltages from around 6V to 12V and outputs only 5V. But it recommended that you use a voltage of 7V to 12V so that the arduino is getting a nice steady 5V. Under 7V and the output won't be as stable and may be off a bit from 5V. You could put as much as 20V into it but then you start to get heating issues on the regulator. So its best to stay in that 7 to 12 range.