Thanks, Drew, for calling out the author of QEMU and linking to his sight: Fabrice Bellard https://bellard.org/ I didn't know he had done so much! Incredible!
He mostly seems to list his hobby projects on his website, not his paid work (except https://textsynth.com and I don't know what else). E.g. he is CTO of Amarisoft. His actual productivity is greater still.
From Bellard the things I like most are the "toy" projects. Qemu, tinyemu and such began like that.
His list of hobby projects includes QEMU and ffmpeg. I'm sure Amarisoft is a fine company, but mentioning them before those two would be burying the lede.