Main

http://explorevedanta.com/vbc-the-10-main-upanishads/

http://www.realization.org/page/topics/upanishads.htm
http://gita-society.com/scriptures/NINEPRINCIPALUPANISHADS.IGS.pdf

http://en.wikipedia.org/wiki/Mukhya_Upanishads
http://www.indianetzone.com/14/eleven_principal_upanishad.htm
http://hinduism.about.com/od/scripturesepics/a/main_upanishads.htm

http://self.gutenberg.org/articles/eng/mukhya_upanishads
http://vedantaupanishad.blogspot.in/search/label/108