mtPaint has pretty poor usability compared to MS Paint.
ReactOS has its own Paint, I wonder... it may be simple to compile it through winelib, or just run it directly via Wine. That'd be a pretty quick and legal way to get a functionally-identical clone.
I think the non-web app that comes closest to MS Paint in functionality is GrafX2. Only other paint program I've seen that has selection-as-a-brush. http://pulkomandy.tk/projects/GrafX2
The interface is a bit different than MS Paint but it has these absolutely wild features, like colour cycling, where the colours can change while you're painting, which can be fairly psychedelic. GrafX2 is more of a clone of DeluxePaint for Amiga.
I disagree about usability. Probably most people just used MS Paint first. MS Paint has different tools and different brush-tip (widths) for pencil/brush/spray-can/eraser that all do essentially the same thing (okay, the eraser's right click mode is different, but hardly anyone knows about it), and then the line/curve/box tools all have _their_ own width. This is all unified into one global brush/pattern option in mtPaint that everything uses. The zoom levels are better too. I think the only thing that's not obvious in mtPaint is the eyedropper.
The one thing I like in MS Paint is the controls to resize the canvas near its edges. Most other programs make you go through a dialog or select and then trim to selection.
ReactOS has its own Paint, I wonder... it may be simple to compile it through winelib, or just run it directly via Wine. That'd be a pretty quick and legal way to get a functionally-identical clone.