Mechon Mamre
Mechon Mamre: Shemini Atzeret and Simkhat Torah
Discusses Shemini Atzeret and its relation to Sukkot and Simkhat Torah. Also discuses the Torah in relation to the holiday. Provides lists of dates on which Shemini Atzeret falls.
Other
Basic Judaism: Jewish Holy Days
This resource provides a great description of the various Jewish holy days. Read about the various holy days, when they occur every year, what the purpose of each of the holy days is, and how these special days are celebrated.
Other
Jewish Holidays: Sukkot, Shemini Atzeret, and Simchat Torah
Defines Shemini Atzeret and places it in the context of Sukkot.