Google Translate - Google's free service instantly translates words, phrases, and web pages between English and over 100 other languages.
WordNet - WordNet® is a large lexical database of English.
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.
WordWeb - One-click lookup in any almost any Windows program; Hundreds of thousands of definitions and synonyms; The latest international English words; Works offline, or reference to Wikipedia and web references.
Wiktionary - Open Source wiki-dictionary by the Wikimedia foundation
Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...