Xenomorph is OpenSource software running on Unix/X11 systems. It provides a unified concept of polynomial and morphologic image filters. It is built for precision, not for speed, that means it works in floating-point mode internally.
It can read PNG, JPG and many other formats, and it writes PNG. A nice feature of this software is it's upgradeability by adding new filters. You can learn here how to do this.
The software is built with the Trolltech QT3 widget-set, which means Windows and OSX- versions are possible, but the port would cost money (Trolltech provides his free license for UNIX OpenSource development only) so it's not available right now.
Xenomorph is released under the Gnu Public License (GPL), which basically means it is allowed to use and distribute it for free (and only for free).