Digg is good, but also some odd ones like ZooLoo and YouMob bring traffic. Reddit is also good, and if Xomba (which share revenue) would stop requiring fifty word blurbs as a price of entry, they'd be good. StumbleUpon seems okay, except I get a feeling they have some internal crawlers that make it seem like your generating more than you are.
Finally, Facebook, while not officially a bookmarking site, brings me more traffic than any other site when I post to my profile. I also get more feed back and followers from Facebook than anywhere else.