Logic Level Converter
When trying to interface your 5v project with a 3.3v sensor or device like an XBee, you'll have to convert the logic level to don't fry it.
Sparkfun has a cheap Logic Level Converter ready to use but if you need it fast and don't want to wait, you can build your own.
BOM (Bill Of Material)
1 x 2N2222 NPN transistor(It's possible to use another NPN transistor but is untested)
4 x 10k Resistor