Michael Henke

This plugin provides flattr-buttons for comments on your blog if the comment author entered his Flattr-ID. 
There will be an additional input field right below the comment text box. Alternativly you can input a piece of code in your theme to place the field somewhere more nicely.

Flattr Comments

This plugin provides flattr-buttons for comments on your blog if the comment author entered a Flattr-ID.

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 Comments vs FlattrButton standalone