I've been using GymBuddy for about 6 months now and My friend likes iFitness.
GymBuddy has really helped me stay on track. Who can remember every weight you use on 30 different exercises? The apps show you what you did the previous workout so you can try to beat your previous workout bests..I hate when I start off too heavy, that ruins the exercise for me.
Gymbuddy also has a 30,60,90 second timer, which helps me in between sets..
What are your Fav Gym apps?