Galaxy Zoo lets you help astronomers explore the Universe. Participation is easy: once you sign-up, and complete the tutorials, you can assist with classifying images of galaxies. You are shown an image and asked a series of questions about it. Your answers are actually important to science!
Believe it or not, humans are much better at this task than computers. To date, more than 150,000 members of the public, inspired by the opportunity to be the first to see and classify a galaxy, have helped professional astronomers via this on-line mass-participation project to carry out real scientific research.