A service for sharing website/links. Here is the description from the site.
“StumbleUpon is an intelligent browsing tool for sharing and discovering great websites. As you click Stumble!, you’ll get high-quality pages matched to your personal preferences. These pages have been explicitly recommended (rated I like it) by friends and other SU members with similar interests. Rating these sites shares them with your friends and peers – you will automatically ‘stumble upon’ each others favorites sites.”
Looks quite interesting have to explore it further.
Rajan
mtrajan.com 
Stumble upon is a really interesting feature. The Stumble upon extension on Mozilla/firefox is really good.