Syed Balkhi of WP Beginner and Thomas Griffin, maker of Soliloquy, just teamed up to create Floating Social Bar. It's a plugin that utilizes the Socialite.js script in combination with a sticky bar for displaying social sharing buttons. Socialite.js is nice because it only loads the heavy social buttons when activated by hover.
It was interesting timing, because I was looking for similarly functionality just recently with Jetpack. I tested the plugin locally just now and it seems to work as advertised.