StumbleUpon is a free web-browser extension that acts as an intelligent browsing tool for discovering and sharing web sites.
Delicious is a free online bookmarking service that lets users save website addresses publicly and privately online so they can be accessed from any device connected to the internet and shared with friends.
Digg delivers the most interesting and talked about stories on the Internet right now. The Internet is full of great stories, and Digg helps you find, read, and share the very best ones. It’s simple and it’s everywhere: visit Digg on the web, find it on your iPhone, or get the best of Digg delivered to your inbox with The Daily Digg.