Hi! I bought an accelerometer for my arduino and I noticed it has two power pins; 5 and 3.3. Is there any difference between them or are they just there in case you've only got one of the specified supplies?
It depends on what breakout board the accelerometer is mounted on.
It's a MMA7361. On the blue board with 10 pins. One of the first pictures on google.
Not much documentation on the blue board. I found a little on this page:
It says "This module's onboard voltage regulator allows for convenient operation at either 3.3 volts or 5 volts". From this I would conclude that the 3.3V pin is the OUTPUT of the 3.3V regulator and the 5V pin is the INPUT to the 3.3V regulator. You can power it with either one but I expect you will get 3.3V outputs.