I am testing Sociable on this blog.

Sociable is a plugin which adds social media buttons to your posts, and does so easily, and beautifully.

At present you can choose from 99 different social bookmarking sites, far too many for me but it means you can pick the ones that are relevant for your blog.

The plugin is also available at The Sociable Plugin.

Update
I wanted a better tweet-functionality so I am testing Tweet This plugin for WordPress

For current plugins at this site: see WordPress Plugins I Use.