Delicio.us and StumbleUpon. Digg.com is presently the leader in social surfing, but I've found a tool that I think is preferable to the others: Diigo. "Diigo" is an abbreviation for "Digest of Internet Information, Groups and Other stuff." (Don't blame me--I didn't name it.)
The Diigo toolset includes tools that not only permit you to save and share bookmarks, but also to highlight sections of Web pages, add your own annotations, and mark your bookmarks with tags. You can make all of this information accessible to others, which turns Diigo into more than just a social networking site or bookmark tool. For those who wish, it can become a research and collaborative research tool, perfect for knowledge sharing and gathering.
You can find my Diigo page at http://www.diigo.com/profile/augieray, but don't expect much right away. I just signed up and am building my bookmarks, tags, and annotations.
Check out Diigo.com and let me know if you like it or if you've found a different social surfing tool with better capabilities.