Social bookmarking is a great way of getting visitors to your website. del.icio.us is the leader of social bookmarking sites. StumbleUpom.com and reddit.com also can generate significant number of traffic. Digg.com can give a flash of traffic (although it is bookmarking site rather a news and video sharing site).
Here is a small list of top social bookmarking sites:
~Enam >> www.enamul.info