Like the ever popular and powerful Pablodraw[0], this is a .Net application.
I tried a few CLI ANSI and ASCII editors on Linux recently, and it was not a great experience. Most are fairly inscrutable with limited documentation, and at least on my OS, some were just plain broken.
REXPaint is not based on .Net, and works flawlessly under Wine on Linux. Although there's no native build, there are a bunch of users on both Linux and OSX (especially since a sizeable portion of the roguelike dev community uses non-Windows platforms).
I tried a few CLI ANSI and ASCII editors on Linux recently, and it was not a great experience. Most are fairly inscrutable with limited documentation, and at least on my OS, some were just plain broken.
Ah, bitrot.
[0] http://picoe.ca/products/pablodraw/