What is Stumbleupon?

Stumbleupon is two things, really:

    1. A social bookmarking site. Like Delicious, Instapaper, and Google Bookmarks, it allows you to save links to articles for later reference and share them. The “Stumble!” toolbar also allows you to randomly discover new articles, based on your interests.
    2. A social voting site. Similar to DIGG and Reddit, it allows you to vote for videos, blog posts, images, and articles you find on the Internet.