I've been on the World Community Grid (WCG) for about a week now and I'm using my computer's idle time to help out with the "FightAIDS@Home" project. I also joined a team to help contribute more computing time.
The premise to WCG is to use computer idle time (like when you go out for that extra long coffee break and still file it as "productive thinking time" in your timesheets haha) for grid computing. According to the WCG website, "grid computing joins together many individual computers, creating a large system with massive computational power that far surpasses the power of a handful of supercomputers. Because the work is split into small pieces that can be processed simultaneously, research time is reduced from years to months. The technology is also more cost-effective, enabling better use of critical funds."
More than the thought of helping out with such an ambitious project, I'm totally geeking out on the idea of how technology is used on a massive scale with so much participation from the average Juan. Pretty darn cool. All you need to do is download the software that makes this all possible.