Croogo-Flattr-Plugin is a project mainly written in PHP, it's free.
A Croogo Plugin to add Flattr buttons to your posts
For Croogo 1.3.2
Install:
Styling:
The Flattr Helper (flattr/views/helpers/flattr.php): I first coded the FlattrHelper as standalone helper. It still can be used as such and therefore has a lot of comments in it. So if you are curious, you can read the whole thing. ;)