Michael Henke

This plugin is including the Flattr button for your blog inside a widget. 
This way the button can be easily positioned in any of your themes sidebars.
Your flatter credentials are all automagically imported from the official Flattr plugin.

Flattr Widget

This plugin is including the Flattr button for your blog inside a widget.

A standalone plugin for integration with the Flattr service for sharing cake on the internet.
FlattrButton works on its own as a single installed plugin. It currently shares options (for easy migration) with the original Flattr plugin (by flattr.com) and FlattrWidget (by Donncha O Caoimh) such as default Flattr UID, language etc. Separate post option changes also affects these plugins, but this will change.
FlattrButton is dual-licensed Affero GPL v3.0 and GPL v2.0. The source code is available by checkout from the WordPress Plugin SVN repository:
FlattrButton will not work correctly without an account on flattr.com. Having an account there means you can flattr me too.

FlattrButton standalone

FlattrButton standalone lets you implement a Flattr button alongside a post, page or as a sidebar widget.

Flattr Widget vs FlattrButton standalone