Add to Any

Add to Any allows you to fully customize the buttons with CSS. Further, you can reorder and add new services, and track data with Google Analytics support.
Add This

Add This features an independent analytics solution to monitor sharing. It supports over 50 social sites, all of which are customizable.
ShareThis

ShareThis allows you to pick from text, image, or rotating button options, with a customizable header and link colors. You can easily integrate this tool into WordPress, TypePad, and Blogger.
Socialmarker Bookmarking Button

This tool adds social share buttons after the text of each article, and automatically fills in the Title, Link, and Text field.
Onlywire

Onlywire is a little different in that it is an automatic bookmarking tool. It’s nice in that it saves time, but many sites will ban your accounts, and the integration with Digg and Reddit doesn’t really work. Also, Onlywire requires the button to be placed on the website to stay in the program. If you use the button, you can customize title, text, and the url of your submission. There is a WordPress plugin also available.
Bookmark Me Plugin

This is a WordPress plugin inserted via editing the template.
Social Bookmarks Plugin for WordPress

This plugin creates XHTML compliant buttons after each post, with fully customized services. You can choose to exclude specific pages and posts from the plugin if you so choose.
Share it!

This is a JQuery Plugin for the Javascript-inclined. It features the ability to toggle and hide a group of social bookmarking buttons with a cool slide effect.
Sociable

A WordPress plugin with a nice backend that allows you to customize priority and inclusion of services.
SocialMarking

Includes a Google Gadget, buttons, and text links.
I hope these buttons get you started on adding sharing features to your website. If you found it helpful, you should RSS to be the first to get designer and developer resources. Or, keep this page handy by bookmarking it on Delicious.