Columbia Encyclopedia, 6th edition - 50,000 article entries, 40,000 bibliographic citations, and over 80,000 cross-reference entries.
Information Please - Online Dictionary, Internet Encyclopedia, & Almanac Reference.
Merriam-Webster - Online Dictionary.
Roman Emperors - De Imperatoribus Romanis: An Online Encyclopedia of Roman Emperors from Augustus (27 BC-AD 14) to Constantine XI Palaeologus (1449-1453).
Stanford Encyclopedia
of Philosophy - a dynamic encyclopedia, each entry is
maintained and kept up to date by an expert or group of experts in
the field.
Statistical Abstract of the United States - PDF format requires Adobe Acrobat (R) Reader.
World Factbook - published by the Central Intelligence Agency (CIA).