Dictionary.com
A good all-purpose dictionary with links to Roget's Thesaurus and dictionaries in other languages.

Encarta Concise Encyclopedia
This concise online version of Encarta provides over 16,000 free articles, but like the Columbia Encyclopedia, it also offers a pay service for more information.

Encyclopedia.com
This Electric Library service provides basic, short articles on many topics with a searchable or browseable database. Also cites magazine articles of interest.

Encyclopedia Smithsonian
This encyclopedia from the Smithsonian Institution features answers to frequently asked questions about the Smithsonian and provides links to Smithsonian resources from A to Z. Most helpful in the areas of science, social studies, and history. Searchable by alphabetical index.

Got Brainy
Visual Dictionary.

Hyperdictionary
The old Exploding Dictionary renamed, with great tools.

Image Dictionary
A chrome extension that enables an image pop-up when you hover over a word.

Little Explorer's Picture Dictionary with Links
A basic dictionary/encyclopedia for beginners.

Merriam-Webster Online
The famous collegiate dictionary is here free for your use.

Rhyme Zone
Writing a poem? Need help with a rhyme? This is the place for you! It also includes links to quotations, the bible, and Shakespeare.

Snappy Words
Visual dictionary and thesaurus.

Visuwords
Shows relationships between words.

Wordsense

Wordsmyth
A combination dictionary and thesaurus that's excellent.