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.
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.