Hard to tell what you think 'cool' is. Unlikely, though for me it was cool, is ReactOS.<p>Its a full, from the ground up operating system that is meant to be binary compatible Windows XP.<p>I worked on it for 3 years, and I can tell you that even though the group of active developers is small, the people are great and there is a large of amount of people watching you (future users?). I thought it was cool because it is huge (code and complexity wise), there is a wide range of things to do, and it can teach you so much. I think I learned more working on this project for 3 years then I did from getting my degree at top university. Just remember, it will never bring you fame or fortune, what I learned took me to a job at Microsoft and now at a startup with a lot of traction.
How about any of mine?
<a href="http://github.com/dylanjs1030" rel="nofollow">http://github.com/dylanjs1030</a><p>I've got a range of different things there. Of them, I think Gun.io's LightWrite is a fairly practical project.