StumbleUpon is a social bookmarking service that operates from your web browser’s toolbar. Just click the Stumble! button and you’ll be sent to a page relevant to your user profile.

StumbleUpon uses different channels to make sure you can stumble upon something you’re interested in reading, as well as a learning algorithm to learn your preferences. Users submit stories and also rate them with thumbs up or down.

A small portion (less than 2%) of the “stumbles” leads to paid advertising matched to your preferences. Paid accounts have the option to turn this “feature” off.

There is also a recently launched StumbleVideo service.