Pinta

https://launchpad.net/~pinta-maintainers/+archive/ubuntu/pinta-daily (Linux)
or https://pinta-project.com/pintaproject/pinta/releases (Windows, Mac, Ubuntu, BSd.. etc)
Preview version (pre-release) 1.7x
Current version 1.6.3...
Maintainer is Cameron White (Canonical)
I recently received responses (March 2017) to crashes & bugs in the program from him and was able to resolve several issues!
Yes, it is being actively developed.
NO, it is not a drop-in replacement for Paint.net (Windows version) NOR claim to be a GIMP replacement. It does have layering, which simpler paint type programs lack, but is not as full-featured as either GIMP or Paint.net, and not all features work in all versions (which is why it is still under development for improvements!)
[Edited by fixitmanarizona, May 29]

I really liked Paint.NET, but was saddened to learn that it wouldn't be supported on anything other than Windows; I guess it has something to do with depending on the .NET framework? Pinta is the closest you can get to a Paint.NET experience on Linux. However... I may be a very unlucky fellow, but Pinta occasionally has some noticeable slowdown while using the program. Whether it be changing a tool or opening a new dialog, Pinta intermittently stops to catch its breath before doing what you wanted it to do. I don't think it's my computer hardware, as other more intensive programs run fine. Other than that little issue, Pinta is a nice program to use.

I decided to try Paint.NET in a virtual Windows 7 machine to compare performance. Paint.NET was more responsive than Pinta.

[Edited by TerrifiedTyphlosion, June 18]

Piece of junk saved a corrupted version of a file I was working on as .ora and only two layers out of 5. I lost 4+ hours of work. Crashes when you so much as sneeze. Use Krita instead, much better interface and works better too. Used on Ubuntu MATE.

[Edited by ghylama, December 01]

This is very very similar to Microsoft Paint. the tools are the same and layout is the same. it is also very lightweight and fast to open. it supports layering which is a big plus. I think every linux should be shipped with this. it comes very handy every now and then when you take screen shots and want to annotate them quickly.

It is great and simple Application I try it on centos 7 and on Ubuntu 16.04 without any problems.

Crashed while trying to open a PNG file => uninstall Pinta