You can do it on the command line with noscl and nostril, but I don't know of any others.

Used to host POW as a service on https://powrelay.xyz/work.html but no one was buying so it's broken right now. Working on a powbot you can tag to buy pow for your events, hopefully that will be more popular. 😅

I have a client that does.

Recently stopped development though, so it's unlikely to ever become public.

POW is only like a hundred lines, I'm not sure why other clients don't implement it.