If you need to count calories, try the My Fitness Pal app. I have been using it every day since January 7th, and have lost 15 pounds. It makes it so easy to keep track of everything you eat.
I love this app. too. I used that for a long time last year. I've recently joined WeightWatchers online & their app is horrible compared to My Fitness Pal. They are both based on the same premise, but WW is not even close to being as good (and that one cost me $59 for 3 months).
My favorite apps are facebook, Scrabble (but mine was about 2.00, not free), WordPlay, IMDB (I always have a tough time placing actors and this helps a bunch), TMZ.
My five year old is hooked on My Angry Birds (99 cents - she grew out of the Angry Birds Lite) ds, Cut the Rope (free).