I've installed and tested a number of plugins. I wanted something simple that could be integrated above and/or below posts or even pages. Almost all I tried were bloated with things I don't need and have floating sidebars that I don't like.
Hupso is simple and shows the major social networks, and it's easy to configure.
I even had a small issue with the Send button (Facebook) that conflicted with the sidebar used in my theme, and the author of Hupso gave me a small snippet of CSS that fixed it.
It doesn't get much better than this for a free plugin.