Once the StumbleUpon extension is installed, Internet Explorer users can begin to
discover cool, interesting, and entertaining websites with just one click. Each
click on the "Stumble" button delivers top-rated websites, news articles, photos,
videos, or Wikipedia pages to the user's browser. Users can then rate each site
with a "thumbs up" or "thumbs down" and share comments on that content. With each
rating, StumbleUpon learns individual users' personal preferences and delivers
increasingly relevant content.
Some have thrown a bit of flak at StumbleUpon, calling them akin to companies like Zango. Considering the very different nature of these two pieces of software and the greater level of user support and user interaction with StumbleUpon, that's really not a fair assessment. If you enjoy site ratings (such as things you see with Netcraft for security ratings), it's a very nifty little tool. With IE support, they are definitely reaching out to a wider audience. Interesting stuff. You can read the full press release in the comments section.