Oxford Reference Online
Published by Oxford University Press, this compilation of over 350 reference works covers dictionaries, handbooks, and short encyclopedias in many discplines, including history and related areas of the social sciences and humanities. Note that items vary in length from short dictionary definitions to 3000 word essays. Longer entries will often include a bibliography of selected references for further reading on a subject.
Credo Reference provides an online reference collection of over 617 reference books. This collection includes various dictionaries and encyclopedias, as well as a broad range of subject-specific titles covering everything from art geography and law to technology.