Galaxy Zoo is great if, like me, you're in to astronomy. It's a project where you sign up for free and categorize images of galaxies. You could potentially be the first person in the world to look upon some of the galaxies you'll be categorizing. All you have to do is mark a galaxy as spiral anticlockwise, spiral clockwise or elliptical.
It's great fun for filling time and the categorization will help scientists gain a better understanding of the build up and distribution of galaxies throughout the universe.
This isn't just a jokey thing that someone set up, it's a genuine Scientific project. In fact, one of its founders is Chris Lintott who is a presenter for the Sky At Night TV program alongside Patrick Moore.
Check it out at http://www.galaxyzoo.org