Apps I have seen/tried/bought:
Weight Track: This is a simple but good app that lets you track your weight. When you enter it every day, it flashes how much you have lost already. 0.99
Interval Timer: Lets you set intervals for circuit or interval work. For example, you could set it to beep every minute, or every minute for a working round followed by every thirty seconds for a rest round. Plays over your music, which is great too. 0.99
iLog it or iTracker: Use these to track your food on any sort of points or exchanges type of program. iTracker also lets you track water, fruits/vegetables, milk and oil. But it is not as pretty, and I find iLog it a little quicker on the data entry side and a little more customizable. Both are 1.99