There are no true protestant groups that are not Trinitarian. Cults like the Jehovah's witnesses are not trinitarian. The Mormons are not trinitarian in any orthodox sense of the word (they are more polytheists). On essentials, true Protestants are fairly unified (i.e., salvation by Grace through faith alone, the incarnation, the virgin birth, the doctrine of the trinity).
To name just a few: Seventh Day Adventists, Church of God, Unification Church, Pentecostals, Unitarian Universalists Christian Fellowship, etc.