It has been long time since I tried, but ... http://code.google.com/p/paint-mono/ http://code.google.com/p/paint-mono/ Then there is http://pinta-project.com/

It has been long time since I tried, but ... http://code.google.com/p/paint-mono/ http://code.google.com/p/paint-mono/ Then there is http://pinta-project.com/ http://pinta-project.com/, that was created basically as a cross-platform clone of paint.net Apart from that, if you want a painting tool, Krita is reasonably good.