ManyThings.org - ManyThings is a multiplatform and user friendly application for students of ESL or EFL to learn the meanings and definitions of different terms and phrases.
Dictionary.com - Dictionary.com is the world’s leading digital dictionary. We provide millions of English definitions, spellings, audio pronunciations, example sentences, and word origins.
InternetSlang.com - InternetSlang.
Merriam-Webster - No other dictionary matches M-W's accuracy and scholarship in defining word meanings. Our pronunciation help, synonyms, usage and grammar tips set the standard.
SlangVocabulary.com - SlangVocabulary.
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.