Last August I reviewed Collin Cunningham's Circuit Sidekick, an iPad app that performs a plethora of electronic computations like figuring resistance and capacitance.
Now, Collin has teamed up with DIY electronics superstars Adafruit Industries to create a similar app, called Circuit Playground:
That last bullet point holds a lot of promise, because Adafruit is one of the most beloved of resources for beginning hardware hackers. And they start you off with a $2.99 coupon for the Adafruit store, offsetting the $2.99 app price. There's also an educational game slated to be included in a future release.
Note that the app is available only for iPhone and iPad right now, but apparently an Android version is in the works. See below for a video demo of the app.