Hipster Logic has released an iPhone electronics reference app, called, well… Electrical Ref. From their iTunes description:
Have you ever wondered how many Ohms that resistor your holding is? Maybe wondered what the capacitance is of a ceramic disc capacitor with the code 103? Forgotten how to assemble a 555 timer circuit? Or wondered how to make your Arduino detect an EMF? Then Electrical Ref is the app for you. It features a resistor calculator, capacitor calculator, & circuit diagrams.
The app costs a buck.