You may have noticed the brand new "Share This!" portlet in the site's sidebar (underneath the Toolbox). This is provided by the ShareThis Extension, which has just been released.
Out of the box, ShareThis comes with a number of popular social bookmarking and news sites - and it's easy to add your own if need be.
Inserting a block of links into a page is also easy, all it takes is a <sharethis /> flag, and you get this:
Special thanks to my good friend Eric Redmond for coming up with some skeleton code and a list of popular sites to start off with.