StumbleUpon is one of the social bookmarking sites that has really made its place this year. Members of it click a “Stumble” button and they are delivered to sites in the StumbleUpon database, and if a visitor gives a thumbs up to a new site, it’s then entered into the collection for other users to stumble upon.
Stumblers can Stumble by category. For example, if they choose Science Fiction, they’ll linked to so many random sites that other members have Stumbled and labeled as Sci-Fi.
You may be having an idea of how this can work to bring visitors to your site. If someone Stumbles you, your site will enter into the StumbleUpon collection, and in the future, others will Stumble across it.
How StumbleUpon Works
You visit StumbleUpon and create an account. Then download the toolbar, which then appears at the top of your browser for easy access. Set up your preferences, i.e. choose the categories of pages you’re interested in seeing when you click “Stumble.” Now, you can rate the site you’re dropped off at with a thumbs up or thumbs down or even review the site.
Those using bookmarking, have a look at this as well