I like this a lot. Every day people are called on to accomplish a specific mission via Twitter.
People submit themselves completing that mission and the results are shown for all to see on http://tweaktoday.com/
New missions are also submitted and voted on by the users.
And there’s a diary of previous missions
Here’s some examples:
Record video of your commute. Compress to 30 seconds.
Take a picture of the coolest building near your work.
Share a photo of the most embarrassing DVD you own.
Simple idea, really well executed.