Fitocracy is a lot of fun, and useful, if you like video games and/or are competitive.
They also have a phone app, which you can use like the website, to log workouts, get your points, and communicate with other members. You add friends, you can compete with other people, create workouts, and connect via forums. I haven't been using it as much lately due to injury, but it's a great site, especially if you lift, to keep up with weight used, and repetitions. You earn points, and level up as you enter in activities.
My username is verloren if anyone wants to add me!