Health apps are a big category for iphone, ipod and ipad apps.
there are loads available, some work and are useful, some dont and arent.
ive tried hundreds of them, and here are the ones I think might be of use to you

nike+
http://www.apple.com/ipod/nike/
iphone / ipod appp
tracks the distance you have gone.
you need a pair of nike shoes with a slot for the nike+ device thing, and a nike+ device thing.
or a normal pair of running shoes, a stanley knife and the nike+ device thing.
useful if your primary means of exercise is walking. not useful if your primary means of exercise is xbox.

dailyburn
http://www.dailyburn.com
iphone / ipod app
dailyburn food app
dailyburn pushupwars
tracks the food you eat, tracks the workouts you do, tracks your body weight
has mini ‘challenges’ for you so that you can keep motivated
has free and pro accounts available
its an excellent place to track everything from.
you can even get bathroom scales that will auto upload your body weight every day =|
useful if you dont have any equipment (you can find lots of bodyweight exercises, etc)
can track walking, running, cycling, swimming, weights, number of situps (crunches) number of pushups, number of chinups, bodyweight, nutrition, lots and lots of things.
if you get a pro account you can track even more stuff.

cycling apps
ive tried a few, but I still think that the best cycling app is the built in google maps, or the motionx gps app.
bmi apps – numerous apps like this are available, you need them to calculate your bmi in order to find out what percentage of the foods you should eat.
smoking counter apps. lots available. help you track how much you smoke (if you do)

these should keep you going for a few months and should keep you fit in the process =)
if you have any other apps that you use on a regular basis then id like to hear about them in the comments so I can try them out =)