Yep, Vim is the first thing to come to mind. I'm not sure there's any program with better undo-ability. I also am not quite sure what the post above means by

Yep, Vim is the first thing to come to mind. I'm not sure there's any program with better undo-ability. I also am not quite sure what the post above means by "GUIs support undo". Yes, most apps,e g., on Windows, support CTRL-Z for undo. But this functionality depends on each app implementing its own undo, nothing OS-wide, and results will vary. I'm not sure what there is in GUIs that supports "undo" other than an app/user convention like CTRL-Z, which hardly seems to rise to the level of "supports undo".