Credo Reference - over 550,000 articles from over 100 reference books. While not designed to provide in depth information, Credo's short articles and definitions will help build your knowledge base. Also provides images, videos, and current news.
Oxford Reference Online - "offering integrated, consistent coverage of a broad range of subjects: everything from General Reference, Language and Quotations to Science and Medicine, and from Humanities and Social Sciences to Business and Professional."
Oxford English Dictionary - is the preeminent authority on the English language. Definitions, pronunciations, and etymologies for over 500,000 words.