There is a nice free service called Addthis
If you want to include the button below each post in blogger (as it appears on this site), select Template -> Edit HTML, then copy-and-paste the code below into the template, right after the following tag.
<div class="post-footer">
Note: be sure to check the 'Expand Widget Templates' checkbox first. When you are done, click 'Save Template'.
If you can't find the tag, copy the template code into a text editor (e.g. Notepad, etc), and search for the 'post-footer' string. You might also want to save a copy of the original template before changing it.
Important: Addthis might recommend to add the script automatically as a widget. Do not do it as it will not work.