The Ancient Middle and South Americans. Though the Spanish found a number of long-depraved societies, their ancient forebears (through more recent analysis) have shown indications of much deeper thought. These indicators include the use of a calendar and eras of time that bear comparison with the Hindu yugas.
The Pythagoreans - who may well have had links with the Hindus (Brahmins) and Celts, and studied Ancient Egyptian sciences.
The Kabbalah - A Judaic creation, and concerned with a pattern by which man may grow spiritually by action on himself. In some ways akin to the sciences of Buddhism, in my opinion.
A Mysticism Index, which also links to the phenomenal Mystics' Who's-Who. See also the Mysticism in World Religions site.
About Theosophy and Helena Blavatsky.
The Gurdjieff International Review — a source of informed essays and commentary on the history, writings, and teachings of George Ivanovitch Gurdjieff. These pages include details about P.D. Ouspensky.
Jonathan Cainer is a leading British astrologer whose site gives mystical insight into this topic and a plethora of related issues written by him and esteemed colleagues. Click here for his page.