I cannot describe how much I love this program/application/web tool, whatever the hell it is.
I discovered StumbleUpon many a year ago, and I can personally say that it changed my life.
How it works: Basically, you sign up for an account and choose from hundreds of categories that you're interested. Based on what you choose, Stumble will take you to pages on the internet that you have likely not seen before.
Even better, when you're just not in the mood for looking at pages about your religion or something, you can choose what specific category of pages you want to browse through.
Over the years I learned that if you just go to a page that you didn't stumble upon and like it (sometimes you'll be making a discovery, sometimes not), the program will add it to the queue of what kind of pages to throw at you. It's pretty important to let it know what you do and don't like, by the way.
Also, if you already have Firefox (which is totally the best internet browser in my opinion and you should definitely get it if you're using something lame like IE), definitely get the stumble upon toolbar. It'll always be there, so it takes less effort for you to start stumbling. I have it and love it!
So in sum, if you want to learn stuff or just see different content on the net with minimal effort, get this. I can't tell you how many hours I've spent stumbling late into the night and how much this has improved my life.