The StumbleUpon toolbar is a must to have in your browser. If you dont have it, you are missing some fun of internet.
Apart from using StumbleUpon for surfing, you can also use to meet like minded people.
StumbleUpon is also cool, if you want to surf through the numerous videos that have started flooding on the website due to the popular emergence of social video sharing sites like youtube and googlevideo.
Other popular choices include to surf random news, pictures, blogs, wiki etc. You can also write reviews of these pages/sites which other users can read.
Currently there are above 7 million users of StumbleUpon.
Read more about StumbleUpon and how it works, here.
Due to high popularity of StumbleUpon, there was a rumor that eBay is going to buy StumbleUpon for $40-45 million in 2007 when total number of users were 2 million.(not conformed though)
Happy Stumbling!