I have been using Last.fm for a long time now, but I have always avoided StumbleUpon because I have been lazy and presumed that I wouldn't want it.
Today I installed the Firefox extension to see what it was like.
To put it mildly, I was impressed. After telling it what I was into, it was able to give me websites that I thought were really interesting and useful, though there are some which I disagree with subtly - too subtly for it's preferences engine to understand.

After reading about the deadly sins of Web Design, and subsequently counting how many I had been forced to do in the last six months for college, I have set out to create my own list of deadly sins.