RunKeeper. RunKeeper is a free application (for Android and iOS) that tracks distance and speed for various activities, like running, walking or biking. If you have a Pebble, it displays the average pace, time and distance on your watch.
Being able to look down and immediately see the pace was surprisingly motivational, and the distance display should be useful as I try to bump it up. This way I can just bike around until I hit my target, and then bike home. It's also nice that the middle button on the Pebble pauses and unpauses the activity without needing to take your phone out and unlock it (pausing is useful if you take a break, or stop somewhere like the library).
Of course, a bike computer only costs $20, so this alone isn't a great reason to buy a Pebble, but if you already have one, it's pretty convenient. I'm excited to see what other applications come out to make the Pebble even more useful.