Sieve is a mail filtering language standardized as RFC 3028 / RFC 5228 and its extensions. libSieve provides a simple, clean API for parsing and executing Sieve scripts. Code is on GitHub: http://github.com/sodabrew/libsieve
Gimp User Filter is a plugin for The Gimp (http://www.gimp.org/), originally written by Jens Restemeier back in 1997. This project is about maintaining the plugin, keeping it compatible with current versions of Gimp, and eventually extending its features