I'm a bit conflicted about the best way to handle these kind of things, but I thought it might be good to collect the existing MO "memorial" questions in a thread here on meta, in case some decision is reached (e.g., to give them all a certain tag). Here are some I've found:
Which of Quillen's Papers Should I read?
In memoriam Torsten Ekedahl [This one concerned an active MO user and so might be considered somewhat different.]
Wikipedia story about Bill Thurston's death
John Nash's Mathematical Legacy
Maryam Mirzakhani's works [Whoops, this one was not actually asked after her death, only edited to reflect that.]
Mathematical work of Jonathan Gleason [this one, posted January 2018, is deleted; it actually concerned a graduate student, actually active here, who had no publication or preprint or at that time, and neither now as far as I [YCor] know. As the post is deleted I copy it here: I heard the sad news that Jonathan Gleason passed away this week at the age of 28. In the spirit of the post on Quillen, I want to ask what are the mathematical contributions of Jonathan Gleason. Somehow the Berkeley mathematical department did not even contact his family after mountains of teaching work he has done. But his work survives and I think it is time to us to have a better appreciation what he has achieved. After deletion OP complained in this MetaMO question, which was answered by a moderator.
(On Meta: migrated here while initially posted on MO) Swinnerton-Dyer passed away?
Jean Bourgain's relatively lesser known significant contributions
Hassan Akbar-Zadeh's mathematical legacy
Ron L. Graham’s lesser known significant contributions
What do you like in the mathematics of Vaughan Jones? And how Vaughan Jones liked mathematics to be?
If you know of more examples, please add them here. I tried to put them in chronological order.
