I use:

* The Hundred Pushup challenge, to force me to do pushups 3x a week
* Fitbit, to keep track of my daily steps

I used to use RunKeeper and Couch25k, but had to stop running so haven't used those in a while.

I was skeptical of fitness apps, but they've turned out to be pretty useful! I may start logging my intake with MyFitnessPal...

Do you use any fitness apps?