I run World Community Grid on my processor and Folding@Home on my HD3850. I average 5222ppd for WCG and no clue on F@H. 174k and 49k points total respectively. I'm much more dedicated to WCG--to me, the work makes more sense. I run almost exclusively the Help Conquer Cancer work units, since that's an issue I feel a strong connection to. As for F@H--I don't pay for my electricity directly, so I just don't want the power to go to waste. I run my computer 24/7 as it is--so it's quite a bit of time taken and work done.
www.worldcommunitygrid.com

And it's popular because people feel a connection to the issues being researched by distributed computing. If I could, I would gladly spend tens of thousands of dollars to buy computers exclusively for crunching. I know a few guys that have spent tens of thousands on such units--including my team's crunching powerhouse and forum admin, DD_TUNG, who ranks in at #4 in the world for points, #6 for work units (a bit over a million units), and runs approximately 311 systems since Sept 2006. I've seen some pictures of his crunching castle--he uses his commercial offices to host dozens of P4s (front-end machines), AX2s, C2Qs, and Xeon DP systems (in racks). The Top100 in WCG are a whos-who of the XS.WCG folk. My personal friend, Movieman, has spent tens of thousands on DP Xeon machines. Right now he crunches with a few Sossamans (Merom-based DP Xeons), 2-3 Clovertowns (Kentsfield DP Xeons), another few Harpertowns (Yorkfield DP Xeons), a single C2Q Q6600 @ 3.6GHz, and now his Core i7 EE 965 @ 3.83GHz on air. Hell--I'd love to have some of his rigs.