Wex. A combination legal dictionary and encyclopedia from the Legal Information Institute at the Cornell University Law School.
Nolo.com Free Legal Encyclopedia. Articles on a wide variety of topics.
American Jurisprudence 2d (AmJur). A detailed and comprehensive encyclopedia of the law, with lots of citations to facilitate research. AmJur is in Lexis-Nexis: Search by Content Type --> Legal Reference --> Advanced Options --> American Jurisprudence 2d.