Surpassed 1 million downloads Rated 5 out of 5 stars
We've had Shareaholic installed for months and there's a reason they've surpassed a million downloads and also just raised angel funding (http://www.techcrunch.com/2009/09/25/browser-plugin-shareaholic-passes-1-million-downloads-closes-seed-funding/) - Their add-on is very useful and smoothly integrated.
Shareaholic currently offers over 60 services for sharing, which will probably cover most everyone's needs. They also enable the ability to place the sharing icon in any combination of the URL bar, the context menu and/or the Navigation Panel. Customizing the services and changing the appearance could not be easier.
This is one that you could install and use every day.