Register your website here: www.stumbleupon.com/
I have gotten many times more hits over the past week than I have had since I started my site in november. Traffic has gone thru the roof.
The more positive reviews you receive, the more people that stumble onto your site.
It's a great tool.
Some of the reviews have been interesting. They range from the "holy crap" kind, to the "This guy is nuts" kind of reviews. It does expose people randomly to info they never would have received otherwise. The vast majority of reviews are positive, and even the negative ones are useful when they are constructive. (one bitched about the formatting, and he was right. So I spruced it up a little)
It's just something a webmaster might find useful.