That depends a lot on what version of MS Paint you want to replace.

For the old pixely version there is a clone: https://github.com/1j01/jspaint

A good alternative for that old pixel paint is https://mtpaint.sourceforge.net/, which focus is mostly pixelart and having a simple interface. It’s not as extreme and complex as http://grafx2.chez.com/

@cynthia
https://en.m.wikipedia.org/wiki/GNU_Paint is closest, but you might also look at https://www.pinta-project.com/