The official Flattr plugin does everything great nowadays. There’s no need for a standalone plugin.
A stand-alone plugin for integration with the Flattr service for sharing cake on the internet. Designed to work with newer WordPress and PHP versions, with awareness of the current site/domain/blog/page/article/author. FlattrButton is standalone and works as a single installed plugin. However, it can share variables with the original Flattr plugin (by flattr.com) such as default Flattr UID, language etc. Any option changes made are specifically for FlattrButton and won’t affect any other plugin.
svn co http://svn.wp-plugins.org/flattrbutton-standalone