Pinta is a lightweight paint application for Linux, OSX and Windows written entirely in C# and using the Gtk# toolkit for its user interface. From the FAQ:
Is Pinta a Port of Paint.NET?
Not really, it's more of a clone. The interface is largely based off of Paint.NET, but most of the code is original. The only code directly used by Pinta is for the adjustments and effects, which is basically a straight copy from Paint.NET 3.0.
Regardless, we are very grateful that Paint.NET 3.0 was open sourced under the MIT license so we could use some of their awesome code.
Jonathan further adds that this release is very close to feature parity with the original Paint.NET.