In case you're not totally sure what StumbleUpon is about, let me explain. It's a way of searching the web - or rather, a huge catalog of web pages about a bunch of different topics. You can search by topic, or by all categories, and you'll be taken to web pages selected at random. When you're taken to a page, you can like or dislike it, and this will help the system find better, more relevant pages for you. If you happen to find a cool website or useful article while surfing the web, you can add it to StumbleUpon's catalog so other people can stumble across it in the future. It's a great way to find loads of amazing things without any effort.
If you use Google Chrome, I recommend you take a look at the Chrome Web Store and download the StumbleUpon extension. I'd say it's the easiest way of doing things. My previous post, "Set Your Sights", is about a brilliant video which I would never have found if hadn't been for StumbleUpon. Check it out. If you like it try StumbleUpon for more cool stuff like it. If you don't, try it anyway for completely different cool stuff.