Possibly the app I use most. Lose It! is several things in one: it tracks your calories. You can choose from thousands of preloaded foods (including restaurant and brand name foods), or enter your own if it isn't offered. It tracks your exercise in much the same way, letting you know approximately how many calories you burned on your last run. It offers you a diet plan by telling you how many calories you should consume based on your height, weight, age, and how much you want to lose. And it offers you support through the website, connecting you with other people who are trying to lose weight just like you.
I love this app because it makes shopping easier. I have a bad habit of seeing something I'll really want on the shelf that isn't any good for me, and somehow it makes its way home with me. With this app, I make my list at home, and it's organized by isle. So I go through the store quickly, grab only what's on the list for that store, and I'm on my way. I can also add prices to each item, so I'm not standing around, going back and forth between my calculator and my shopping list, giving me less time to dawdle in the isles.
Couch to 5K
I love this app mainly because it has helped me do something I never thought I'd do: run. Granted, I'm not done yet and I may end up hating this app before I get there, but until then I adore it. They recently added a feature that lets you take notes on that day's work out, which I think is great. Now I can look back in the app and see what was going on each day. I plan on buying the Bridge to 10K next.
Yoga Trainer Lite
I use the Daily Warm up after my C25K training. I run it through twice, once leading with each leg, and love how it stretches me out. They also offer a meditation course, and a whole slew of poses. If you want more courses, you'll have to pay for the full version. This is enough for me, however.