Planet Hunters is a site where you can help hunt for exoplanets (planets that orbit stars in other solar systems). All you have to do is look at an image and answer a few questions that are posed to you by the interface on the website. Here’s a quick tutorial:
What you are looking for is evidence of a transit. That’s where, from your point of view, it looks like something has passed in front of the object you’re looking at. The imagery comes from the NASA Kepler mission. If an exoplanet is identified and confirmed as a result of your analysis, you will be listed as a co-author on the discovery paper.