Distributed Computing:
What is your computer doing when you're not using it? So you leave it on? Is it just sitting there using up electricity? When I'm not using my machine, it keeps working. Even when I am using it, I have it doing useful things. As I write this, my machine is helping out with cancer research.
Computing projects I use on a regular basis so I can justify the existence of leaving my computers turned on:
I first started with the SETI@HOME project because I thought it was a clever idea and a cute if not ridiculous project to participate in. While it isn't the original distributed computing project, it is the most well known. In 2001, a friend told me about the UD project and I thought it a much more worthy cause. I currently run the UD program on my home PC. I occasionally install the SETI@HOME client on UNIX machines if I am not using them or not using them to capacity.
Distributed computing projects allow you to help a cause with little effort. I feel it's worth it, especially if it's something you believe in. And if you leave your machine on all the time, it's a great thing for your computer to do while you're not using it for important things (like video games).