Paint.NET 2.x running on Linux:
http://tirania.org/blog/pic.php?name=paint.net.png&size=large&caption=Paint.NET%20running%20on%20Linux
As for Mac OS X, the Paint.NET's author writes:
May 20th, 2008: Well, I bought a
MacBook Pro. That doesn't mean I'll be writing a Mac version, but I am
approaching the Paint.NET 4.0 design from the standpoint of having much
more platform and UI-toolkit independence ("UI toolkit" referring to
WinForms, WPF, Cocoa, GTK, etc.). Please do not inquire further though,
as we'd just be spinning our wheels with speculation ... I have yet to
write any Mac code, even "hello world". But I'll at least admit that a
native Mac version is desirable.
I have some little experience with accessing Cocoa (one of Mac OS X's native GUI APIs) from C# code and it wasn't easy.
If Paint.NET will run on Mac OS X, it will be the Windows Forms version, I am sure.
Note that .NET 2.0 programs work on Linux and Mac OS X. Most .NET 3.5 programs also work. Programs that use WPF do not run anywhere except Windows. The problem with Paint.NET is apparently that it uses many native Win32 calls.
Using Cocoa from C#:
http://www.netneurotic.net/DNET/StupidWordCounter/
.NET Windows Forms program running on Mac OS X:
http://web.mac.com/ajbrehm/Resources/CocoaSharp.html