If We Digg Together...
Digg is a place for people to discover and share content from anywhere on the web. From the biggest online destinations to the most obscure blog, Digg surfaces the best stuff as voted on by their users.
StumbleUpon helps you discover and share great websites. It uses ratings to form collaborative opinions on website quality. When you stumble, you will only see pages which friends and like-minded stumblers have recommended. This helps you discover great content you probably wouldn't find using a search engine.
del.icio.us is a social bookmarking (collection of favorites) website. The primary use of del.icio.us is to store your bookmarks online, which allows you to access the same bookmarks from any computer and add bookmarks from anywhere, too. On del.icio.us, you can use tags to organize and remember your bookmarks, which is a much more flexible system than folders. In addition, you can share your favorites with friends, family, coworkers, and the del.icio.us community.
Mahalo (Hawaiian for 'Thank you') is a human-powered search engine that creates organized, comprehensive, and spam free search results for the most popular search terms. Their search results only include great links (or links filtered by humans!).