Bartleby.com - Provides access to a variety of reference materials, including The American Heritage Dictionary (Fourth Edition), Roget's II New Thesaurus, Bartlett's Familiar Quotations, and more. These are almost entirely historical editions, long out of copyright.

Infoplease - from Pearson Education, includes free access to the Time Almanac, The Columbia Encyclopedia (Sixth Edition), The Random House Dictionary, a thesaurus, an atlas, white pages, and more. This site is most interesting for it's featured articles of timely content, but be prepared for prominent advertising and pop-ups.

Almanacs

The Old Farmer's Almanac - from Yankee Publishing Inc., the online version of North America's oldest continuously published periodical. Provides weather forecasts, tides, a calendar of the heavens, recipes, gardening tips, and much more. Be prepared for pop-up advertising.

Time Almanac - from Pearson Education. Not a comprehensive resource, but a quick place to search for historical highlights by date or find listings of common historical information.

Dictionaries

OneLook Dictionaries - from OneLook, search more than 675 general and subject dictionaries from this site, particularly unique is the inclusion of translation dictionaries. This site has no pop-up advertisements, but many of the dictionaries linked do have pop-up ads.

Dictionary.com - from IAC Corp., offers multi-source dictionary, thesaurus & quotation search using resources from Random House and Harper Collins

Merriam-Webster Online - from Merriam-Webster, basic dictionary and/or thesaurus with audio clip pronunciation guides. The site has many ads, some pop-ups, but the dictionary search box is easily accessible at the top of the page.

Specialized Dictionaries

Acronym Finder - from Mountain Data Systems, the world's largest and most comprehensive dictionary of acronyms, abbreviations, and initialisms

Dictionary of Medical Terms - from Merriam-Webster, the National Library of Medicine, and the National Institutes of Health, the dictionary is part of the Medline Plus Website.

Rhyme Zone - from DataMuse, find rhyming words, homophones, synonyms, and more.

WordReference - from Mike Kellogg and WordReference.com, dictionaries in 15 languages, including Asian,  Arabic and Cyrillic scripts, also includes English definitions.