An Apple II emulator running Print Shop modified to generate PDF downloads of your prints.
I used this program A LOT back in the day. On a Commodore64 of course.
It doesn’t get more retro than this.

Classic Mac emulator running inside a browser.

Follow up on previously linked item.
This is some technical background explaining how the browser-based emulator hosted by the Internet Archive so you can run old Mac software works.

What's going on here is very cool for those who understand it: an emulator for early Macintosh computers (PCE) was ported to run as JavaScript (!) via Emscripten. And the Internet Archive is packaging it and making it available for free.

Also, I get to play the original Lode Runner after all these years.

An Atari emulator that not only emulates the hardware, but it actually displays the graphics so that they look as if they were coming out of an old school CRT TV. Cool!