My original plan for this page was to have the largest repository of social links on the web. But there are too many and its been done to death so I'll just point you to the social bookmark gurus who have spent lots of time compiling the lists of social bookmarks:
3Spot…
has the most exhaustive list of Social Bookmarking. 3Spot takes it up a notch by breaking down the Social Bookmarking services into different categories:
A social bookmark is a link created by a web user that is categorized and viewed by the public. If you have ever had bookmarks at work and moved to a new job and wanted access to your old links then you will immediately understand the signifigance of the social bookmarks. They offer a way in which users can access their bookmarks from anywhere in the world with an Internet connection and a way for users to have control over the classification of those bookmarks.
Social bookmark buttons on a blog are especially powerful. These are buttons on a blog and/or website that allow readers to submit articles they like to Digg, Del.icio.us, Reddit and other socially moderated bookmarking/news/feed sites at the click of a button. Readers will also be able to add articles to feed readers such as Google Reader, My Yahoo! and Bloglines. This is a great way to advertise as each link from another site (known as a back link) will give more ranking on search engines. It also allows for something known as social marketing optization.
3spots at Blogspot has one of the most comprehensive posts on social bookmarking and code for blogger, wordpress and Typepad/MoveableType!
Reference:
http://www.educause.edu/ir/library/pdf/ELI7001.pdf
http://en.wikipedia.org/wiki/Social_bookmarking
Nature Publishing Groups. Social Bookmarking Tools (I).