I have had a traffic spike on the one of my blogs recently. Most of it have been delivered by the various Web 2.0 communities:
As you can see, almost all (two thirds, to be precise) unique visitors came from the StumbleUpon. It is a best Web 2.0 traffic source. Moreover, it is considerably easy to get the pretty high ranking there.
And for my surprise, a well known and pretty popular Shoutwire social network have delivered seven times less visitors than Polish Digg clone Wykop. Although the story was on the frontpage of both of them for a couple of days.