If you have a blog or website, you can add share buttons by using a plugin such as addtoany or you can create individual share buttons and add them to single posts. Here are the web pages for creating your own custom share buttons for the following services:
Facebook: http://www.facebook.com/facebook-widgets/share.php
ReTweet Button: http://help.tweetmeme.com/2009/04/06/tweetmeme-button/
Digg: http://digg.com/tools/integrate
StumbleUpon: http://www.stumbleupon.com/buttons/
Delicious: http://delicious.com/help/savebuttons
Reddit: http://www.reddit.com/buttons/