I have found StumbleUpon to be by far the best way of generating traffic. It won't work for everyone. Don't bother using it for B2B stuff. But if you create quirky fun content aimed at consumers, it can bring a lot of eyeballs to your site. And you don't even have to pay for ads. Bulk-standard link submissions can work very well.

I have a blog that I have not updated for more than a year that still pulls in 100,000+ unique visitors a month via StumbleUpon.

One of the tricks is that once you have a viral article on StumbleUpon, you can use that article to promote other articles, in the hope that they too will be voted up and become viral in turn.