Thamizhchelvan

Will add bookmark icon for each post and page to share with popular social bookmarking sites.

Simple Social Bookmarking Plugin

Will add bookmark icon for each post and page to share with popular social bookmarking sites.

Scott Grayban

It puts a bar of icons at the bottom of each post allowing your readers to submit your posts to several different 
social networking and bookmarking sites. (del.icio.us, digg, fark, etc.)
The original plugin was called <a href="http://blog.calevans.com/2006/02/08/notable-another-wordpress-plugin/" rel="nofollow ugc">wp-notable</a> 
but no longer worked or was supported. The author was Cal Evans.

notable

Adds social bookmark links to each blog entry.

Simple Social Bookmarking Plugin vs notable