I'm a big fan of DailyBurn for tracking weight and nutrition. While a lot of iPhone users stand by the Lose It! app for that sort of thing (and rightfully so, it's a good one), I prefer to have a service that's accessible from any browser, and DailyBurn (formerly known as Gyminee) is the one I've tried and liked the best. Earlier today, while adding far too many calories to my nutrition record (CES and its various events are not being kind to my diet this week), I noticed that DailyBurn has another app out called Push-Up Wars, designed to help you track your push-up ability.
Just like the main app, Push-Up Wars allows you to enter and register how many push-ups you're doing each day, and then the app will track your progress both among your friends and on a global leaderboard level. Push-Up Wars is notable even if you're not an exercise fanatic, just because it's the first app I've seen that's not a game, technically, but still uses Game Center. You can earn ranks and achievements as you "level up," and then see global and friend-based leaderboards.
Registering push-ups requires you to take a video of you doing the exercise (only uploaded if you agree to do so), so the app requires a video-enabled iPhone or iPod touch -- no dice on older versions. But it's an interesting idea that's executed quite well, and if you want to get serious about powering up those arms, Push-Up Wars might be able to help.