I just set up a Stumbleupon account this semester after hearing so many of my friends talk about it. They talked about how fun and addicting it is. I'll admit that it is definitely addicting, but I was surprised at how much I have been able to use this for educational purposes.
The way it works is you check mark a bunch of interests of yours and then click "Stumble." It takes you to sites that are associated with your interests. You can choose to give it a "thumbs up" and it will keep track of that site kind of like a bookmarking site, such as Delicious, would. It's like being zoomed across a city by a cab driver who knows what types of new places that you'd enjoy. I chose a few personal and fun interests but mostly stuck to educational ones just to see if I could use it for that purpose. It worked! It took me to sites that I could potentially use with my students such as Maps of Wars, Her Stories in History, Have Fun with History, and Breathing Earth. As you can tell, I picked a lot of history related interests... should have been a social studies major, haha. And if you've never been to Breathing Earth, I highly recommend it. It's a great tool to show your students and give them some perspective on life. Of course, whenever I come across a great site, I put it in the bookmarking site I use, Diigo.