I have one more tool to share :)
PhotoFiltre by Antonio Da Cruz
The layout is quite similar to Photoshop and it has tons of functionality (layers, plugins, and much more).
Best of all, there is a free version that has already a lot of functionality and then there is a payed version (about 25�) with so much functionality that I have not missed anything I need for my work.
The Free Version can be found here: photofiltre.free.fr