Thank you @troydavis! Greenhouse and Lever definitely seem to have the lions share (lever skewing older Y Combinator cohorts and Greenhouse newer). If you just happen to come across any newer/ up and coming ATSs, I'd love if you could update this thread (while I keep digging).
(Some companies link to job descriptions on their own site and/or embed the ATS-hosted form in their site, but many of those use Greenhouse or Lever too. The companies linking directly to ATSes do seem representative.)