I recently read an article in the IDEA Health & Fitness journal (May 2012) fitness-related smartphone apps. In the article several health professionals recommended smartphone apps that they like to use and tell their clients to use. I have two apps that I like. The one I use regularly is Map My WALK. I use it every time I walk because it uses GPS and tells me how far & how fast I've walked. You can save your walks and then go to their website to see how much you've been walking. I find it very motivating to see how much or how little I've walked each week or month. If you want, you can enable Facebook and find friends who use the app to encourage each other.
Another app I like is Lose It! If you are wanting an electronic food diary, this is a great one. You can log the foods you eat, and this app will help you keep track of your calories. You can also log your workouts, so that you know how many calories you've burned, too. You can set weight loss goals, and it will help you determine how many calories you should eat each day. It also has social features if you want friends to encourage you along the way.
Both of these apps are available as free apps. I think they are great tools to help you with your fitness goals!