The WeightWatcher plan is one of the most popular diets out there, and it's also one of the few to have proven short and long-term effectiveness. This free app will watch you while you're watching.
The extensive library of foods - over 30,000 - includes restaurant meals and gives a WeightWatchers point value for each entry, as well as calculating your daily point allowance and how many points you have remaining.
While this app really is indispensible if you're following WeightWatchers, it's not much good otherwise, as many of the features can only be used if you have an online subscription, which you pay for.