When I put together my new theme, I made it a goal to integrate more of the social bookmarking websites. The benefit to me is that my articles and website will get more attention. The benefit to you is that you can save / bookmark / share my articles easier.
I've found the most useful bookmarking site to be de.licio.us. De.licio.us provides a very simple API which allows me access information about who has bookmarked my articles quickly. Here's how to pull the bookmark count for a specified URL.
When you receive the data, you can do whatever you want to format it. On this site, I get the count and append it to the "de.licio.us" link at the top of the article. How would you use this data?