The Cardano stake pool from the Buttworld.
I operate the pool. I'm a blockchain enthusiast from Switzerland, with years and years of professional software engineer and DevOps experience. I'm also a BUTT enthusiast - only my wife's though (she is sitting right next to me ಥ⌣ಥ).
BUTT currently uses a variable fee of a maximum of 3%. This is because there is currently a mandatory fixed fee of 340 ADA.
The BUTT is still sustainable at 0% because I put my own butt in the BUTT. I earn delegation rewards just like you do.