Open Source Projects That You Should Know About?

cosyne asks: "Seeing the recent announcement about FreeS/WAN reminds me that there are many cool projects that I'm mostly unaware of (or, more often, completely oblivious to), which I only hear about when Slashdot posts a 'Developers finally giving up on this cool project' story. What projects (software or otherwise) are out there that would benefit from more involvement if only they had the publicity?"