The list of lists
Sarlo's Guru Rating Service - Rich with teacher listings, ratings, opinions, and supported by a very active email forum.
Guide's, Gurus, and God-Beings - "I aim to list and categorize as many helpful beings as I can living and historical. By helpful I do not necesarily mean that I agree with their views. Rather, I feel that it is helpful to explore a wide range of views."
Clara Llum's links, lineages, and teachers - with a great collection of videos.
Lists of Teachers of Advaita Vedanta - A Wikipedia entry.
TAT Forum Index of Authors - Congratulations to TAT on writing this index, which links to articles by each author. Perhaps no other nonduality website is known to possess a useful written index of any kind.
Divine Inspirators - Many Indian sages that few have heard of. Video clips too.
Strictly Spiritual - Includes ratings of gurus and teachers; actively kept up. Good message board. Informative articles.
Satsang Worldwide - Access living teachers by clicking on a map of the world. Very well done site.
Some Teachers of Advaita - Dennis Waite's fine list of teachers with brief commentaries, photos, quotations. An important Advaita/nonduality site.
Germaine's Tapestry - Beautifully and carefully constructed lists of teachers from all times and traditions.
Erica Molnar's list - Interesting. Includes people from popular culture, musicians, literary figures.
The Thai Forest Traditions - Several recent important teachers not mentioned on other lists of teachers.
Spiritual Giants - List of several dozen living spiritual teachers with brief commentary and quotations.
Satsang.nl Links - Good list of many teachers with related material.
When I Awoke - Good reports about half a dozen living teachers.
Living Satsang - Several dozen teachers listed with supportive material.
Connection List - Pretty good basic list of teachers in German with photos.
Guru's Feet - Find your guru ... post your guru.
Lists that do not appear to be kept up to date
Lists of teachers who lived long ago
Arunachala Saints - List of several saints from over the centuries.
The Master List of Masters - List of Zen masters that lived centuries ago.
Before Hui-Neng: The Five Previous Chinese Patriarchs - More people from centuries ago.
Female Buddhas and Bodhisattvas - 32 historical figures, including graphics, supportive material.
Who's Who in the History of Mysticism - Figures in Western mysticism up to about 1700.
Biographies of Saints - Hindu saints from centuries ago.
Zen Lineages - Zen teachers through the centuries.