Nobody really does, which I find extremely frustrating since sieve is supposed to be the Internet standard for mail filtering.
Fastmail sort of does, in that they use sieve rules; but last time I looked, they didn't have a managesieve interface, which means I can't just upload the sieve rules I have spent a lot of time creating on my home email server. I have to create them from scratch using Fastmail's rather clunky (IMO) interface.
As far as I can tell, no other email provider uses sieve at all.