StumbleUpon is the August 2006 Site of the Month on DonationCoder.com Links Page:http://www.donationc...com/Links/index.html
StumbleUpon may currently be the best place to discover new sites on the internet. While much of the charm of digg.com and its clones has faded as they become subject to more manipulation and mindless rating based on headline keywords, StumbleUpon has only gotten better and better as its audience grows. The premise is simple, using a toolbar installed in your browser, you can rate sites you visit with a thumbs up or thumbs down. Or click a button to be taken to a random page rated highly by others who have similar tastes as you. By clustering similar people together, StumbleUpon is able to do a great job of showing you new sites you are likely to appreciate.
Despite it's large and growing user base, StumbleUpon still feels like somewhat of a class project.. It needs a major v2 facelift, and better organization of some of its social networking features. Regardless, some of the features are wonderful, like the ease of sending links you like to your friends via the stumbleupon network or email. We love StumbleUpon and we highly recommend you install the toolbar and give it a try. StumbleUpon makes money by providing an enhanced account feature you can opt to pay for ($20), which is nice but not essential, and by occasionally delivering you a page that has been paid for by a sponsor instead of the usual random page.
I consider myself a StumbleUpon fan now and highly recommend it.
And don't miss housetier's mini-review of it here