Only this small group of colleges/universities require Subject Tests from some/all applicants:
California Institute of Technology
Harvard/Radcliffe Colleges (MA)
Harvey Mudd College (CA)
Massachusetts Institute of Technology
Webb Institute (of Naval Architecture) (NY)
require Subject Tests from all applicants, regardless of what other standardized testing results are shared.
McGill University (PQ) requires Subject Tests from all applicants if they submit an SAT but NOT if they submit an ACT.
Cooper Union for the Advancement of Science and Art (NY) – required for prospective engineering majors only.
Cornell University (NY) – required for applicants to the College of Arts and Sciences and the College of Engineering.
Northwestern University (IL) –three required only for students applying to Integrated Science and Engineering Program—ONLY SCHOOL IN US REQUIRING THREE; may also consider AP, IB or college courses to fulfill requirement.
University of Toronto (ON) – required for applicants to all programs and faculties except for Applied Science/Engineering.
Tufts University (MA) required Subject Tests from some/all applicants in the 2017-2018 school year but no longer requires them:
"Beginning with applicants to the Class of 2023, Tufts requires either the SAT or the ACT. We do not require SAT Subject Tests, the SAT Essay, or the writing section of the ACT; you may submit those scores to Tufts if you choose to sit for these sections, but please note that they are not required and SENDING THEM WILL NOT INCREASE A STUDENT'S LIKELIHOOD OF ADMISSION. This information pertains to the 2018-2019 school year, college Class of 2023."
L’ecole Hoteliere de Lausanne (Switzerland)
King’s College of London (United Kingdom)
require Subject Tests from all US applicants if they do not submit AP or IB test results.