I had a chance to play around with Collin Cunningham’s Circuit Sidekick iPad app, which is an application that assists electronic hobbyists with various formulae necessary for basic electronic projects.
Collin, who shoots the popular Collin’s Lab videos for MAKE, provides tools for determining resistor values (if you haven’t memorized a BBROYGBVGW mnemonic), capacitor values, a data sheet reader, an Ohm’s Law calculator, and more. My favorite tool is the LED Resistor Calculator, pictured above. It helps you build an assembly of LEDS and instructs you on voltage and resistors needed.
At $2.99, Circuit Sidekick is an awesome tool that offers great value, and I recommend it for anyone who wants to get started in electronics and has a hard time grokking the myriad formulae and conventions of the hobby — as well as for those of us who appreciate a utility that saves us time when constructing circuits.