Hi there, just started testing your plugin and noticed that there are a few simple issues that should be fixed to make it compatible with the WP_DEBUG mode in WordPress.
Here's the codex article about WP_DEBUG mode:
When I enable your plugin I get the following errors on all pages (the worst kind, as they stop any forms from submitting properly while WP_DEBUG and your plugin are active)
Notice: Use of undefined constant new_flattrwidget_widget - assumed 'new_flattrwidget_widget' in /path/wp-content/plugins/flattr/flattrwidget.php on line 143
Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /path/wp-includes/functions.php on line 3303
Notice: Use of undefined constant new_flattrwidget_control - assumed 'new_flattrwidget_control' in /path/wp-content/plugins/flattr/flattrwidget.php on line 144
Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /path/wp-includes/functions.php on line 3303
Notice: Use of undefined constant FLATTRSS_PLUGIN_PATH - assumed 'FLATTRSS_PLUGIN_PATH' in /path/wp-content/plugins/flattr/flattr5.php on line 392
The errors should be fairly simple to fix, and the deprecated functions are pretty easy to replace. In testing out the widget I spotted a few more errors that you should probably take care of.
It's a good idea to always keep WP_DEBUG enabled while working on your plugins so that you can kill any bugs that come up as you see them. Overall it can be a pain sometimes but results in cleaner code as well as code that won't get in the way of other developers who use the WP_DEBUG system.