Aw Guo

This plugin will allow users to digg / bury every thread of your comments, thus find out the most valuable comments as well as the human-spammers.

WordPress Comment Digg

Digg or bury your comments and generate a percentage bar in your comment boxes.

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.

WordPress Comment Digg vs notable