I love this Ohm's poster by ZarthCode's Anthony Clay. In addition to the classic Ohm's Law triangle ("It's the Law!" the poster proclaims) it also includes a resistor color chart, a discussion of voltage dividers, as well as an explanation of the markings on surface mount resistors. Just the thing for a hackerspace! The poster and its three siblings (capacitors/inductors, transistors, and microcontrollers) can be purchased on Anthony's site.