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:
In memoriam Torsten Ekedahl [This one concerned an active MO user and so might be considered somewhat different.]
John Nash's Mathematical Legacy
Maryam Mirzakhani's works [Whoops, this one was not actually asked after her death, only edited to reflect that.]
Jean Bourgain's relatively lesser known significant contributions
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.