There are a lot the most popular ones that I've tried are these
Angry Birds (Will keep you occupied for hours)
Doodle Jump (good for passing the time in a one or two minute wait)
Tiny Wings (very addictive)
Fruit Ninja (great but uses a lot of battery power, make sure you have a charger with you if your going to be playing this
You should get Sally Spa... It's phenomenal. As well as Waldo. Next best is Cut the Rope