A frequently asked question here. One recent thread is
http://forum.lowcarber.org/showthread.php?t=479066

There are always newer apps, but I think the most popular ones have advantages. Go into whatever App Store that you use, check Top Charts for the Health Category. MFP is still most popular in the Apple store, free app, followed by Lose It and Carb Manager, a Keto Diet app.

The top paid tracker app is Cronometer, a whole $2.99, if micronutrients are something you also want to track. Jean uses that one, see above thread.