Too web pages about Paint.NET for Polish users.
http://www.paintnet.info.pl (This web page is only about Paint.NET)
Examples of particular pages:
Gradient - http://www.paintnet.info.pl/gradient.html
Layers - http://www.paintnet.info.pl/warstwy.html
List of functions - http://www.paintnet.info.pl/funkcje.html
http://planetslajd.cba.pl/viewpage.php?page_id=6 (This web page is partly about Paint.NET).
These too pages are partly under construction but (I hope) useful.
I am not sure if this post is against the forum rules, because I am the author of http://www.paintnet.info.pl (this is no commercial web page).