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.
GoldenDict - The program has the following features: Use of WebKit for an accurate articles' representation, complete with all formatting, colors, images and links.
Artha - Artha is a handy thesaurus based on WordNet with distinct features like global hotkey look-up...
Lingoes - Lingoes is an easy and intuitive dictionary and text translation software.
WordNet - WordNet® is a large lexical database of English.
StarDict - StarDict is a Cross-Platform and international dictionary Software.