Altimeter for iPhone

If you're an outdoorsy person who like to know the elevation you've traversed when you're out and about, this inexpensive, simple app is a fine tool to have along.

Intelitrainer for iPhone

This excellent fitness app offers all the benefits of a personal trainer at a fraction of the cost.

iWorkout Muse Pro and iWorkout Muse for iPhone

If music is an essential part of your workout, then iWorkout Muse Pro (or the more limited, less expensive iWorkout Muse) should become part of your exercise regimen.