Collaboratively built legal encyclopedia and dictionary from Cornell's Legal Information Institute.

From University of Florida.

From the University of Oregon.

Good for misspellings.

Here is a search plugin for the subscription version of the OED. I'm not sure if it works from outside of the University, but with conquery, it's super powerful.

Three really nice looking extensions, allowing users to 1.double click on a word for the definition as a tooltip 2. download-to, where you select a couple of default locations where you might download files.

Glossary of EBM terms from the Centre for Evidence-Based Medicine

This invaluable resource will guide general readers and students alike through the often bewildering maze of equations and theorems encountered within the mathematical universe. Covering everything in the field, from pure and applied mathematics to statistics, it also covers areas of more general interest, such as fractals, game theory, and chaos.

Articles on writers and countries. Coverage begins with the late 19th century.

More than just a dictionary, this volume contains fairly lengthy original essays.

An important reference work in Economics. New Palgrage includes overlapping entries on many important topics and people in the current Economic thought. Each entry is signed with bibliographies.

Short, clear definitions of terms used in Economics.

This online dictionary features over 3,000 entries. Covering a wide variety of linguistic topics, languages, and more, it's a great, easy-to-use source. It is browsable and searchable.

With 10,500 entries, this is a comprehensive online dictionary. Browsable and searchable. Entries are brief with hyperlinked cross-references.

Brief definitions of social work terms and concepts.

Online version of the twenty-volume printed edition, published in 1989.  Contains almost 300,000 entries, 650,000 word forms, and 2.4 million quotations which illustrate the development of the English language from before Beowulf to the present.

Up-to-date dictionary useful for quick look-ups of philosophical terms.