Social bookmarking sites allow websurfers to save, catalog, and share interesting pages they find online. The Sociable plugin appends links for your readers to use those sites to the end of each of your blog's posts, increasing your potential audience.

Sociable is now maintained by Joost de Valk. For the latest version and any support questions, please visit the official Sociable page on the Wordpress plugin directory.

There's also a fork of it for Wordpress 3.0 in the Wordpress plugin directory, so maybe that's best place to get it now.