Look in to ShareThis, including their advanced customisation options. Systemantics used this on the new Cooper Robertson site designed by Project Projects. See implementation on Bridge Park page. Share buttons always feel like a compromise (they look great but are limited in their usefulness, or they’re so functional but look clunky). Can be a pain and really get in the way of a design, at least here they’re confined and off the main page.
Also, see spin.js (looks useful) and the Dublin Core Metadata Initiative (also here).