I have been using StumbleUpon for about 2 months. It is a free web service and can also be a Firefox add-on so a toolbar appears in your browser. I use Firefox so I prefer the toolbar for ease of use.
How does it work?
When you sign up, you identify categories and interests that you have as well as contacts. Based on that information, the service takes you to random websites that other users have identified as good and applicable to your interests. It's pretty easy and straightforward. And yes, I have run across several sites that were quite good but I may never have found otherwise.
What are the benefits?
This is a great way to share bookmarks as it is easy to send links to specific friends in your contacts. I have used this as part of my PLN (personal learning network) to trade links on several occasions. It is also a great way to stumble across some good sites that could otherwise be lost in the exobytes of data on the web.
I see this is a tool for professional growth but can also lead you to some good classroom resources and tools. In addition to finding some resources, it can turn some leisure surfing into productive and applicable professional or personal learning. Who knows...maybe someone will Stumble across the Be Literate blog.