Circuit Simulator for iPad
There are a few, if you are a Spice fanatic then I'd give this one a try first:

Spicy Schematics Free is a trial version of the full iPad app, Spicy Schematics

Circuit Lab HD and DCircuit Lab HD are two fully featured node analysis simulators, the first is an analog simulator in the Spice mould, the second purely for Digital gate and logic combinatory analysis.