For example I know
There is a similar question about developers-only, Are there famous developers using Stack Overflow?.
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Peter Shor is a mathematician and a professor at MIT. He is best known for his algorithm, which allows a quantum computer to efficiently find prime factors of large numbers. This might not sound like a big deal, but once we are actually able to create quantum computers of a decent size, we have to replace (almost) all encryption schemes which are currently in use.
As a mathematician, of course he's active at MathOverflow and Theoretical Computer Science, but it is interesting to see that he's one of the top users at English Language & Usage, which I assume is more like a hobby to him.
No doubt the most famous person that ever had an active account on a Stack Exchange site is frosty aka Ross Ulbricht.
I don't think there is a need to explain why he's so famous, world wide.
The relation to Stack Overflow is explained well here, and it's also discussed here in Did the Stack Exchange staff members assist in the apprehension of Ross Ulbricht?.