There are so many out there but I usually only find myself looking at the top 50 free ones. I don't particularly like playing games so wouldn't find a use for those apps.
One really useful one (found vis my friend Rachael) that I can see myself using everyday is Runkeeper. It tracks the distance covered during exercise - either running or walking. No way am I running so walking it is. I have used it 3 times since last night and I can't see the novelty wearing off of seeing how far I have walked, how many calories burned, the pace I am going etc.
It was quite funny to see the zig zags across the field on the map when I took the dog for a walk.