I had a Mac ROM cartridge for the ST, which basically turned it into a Macintosh. And Atari had a 640x400 monochrome monitor that looked just like a Mac - an awesome hack.
Yep long before there were official Mac clones (remember those?!) the ST was an unofficial one in the form of Magic Sac / Spectre GCR. Never had it myself, but what a glorious hack indeed.