Reverse Dictionary - Describe a concept and get back a list of words
WordNet - WordNet® is a large lexical database of English.
Silicon Valley Dictionary - Urban dictionary meets Silicon Valley
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.
Dictionary Anywhere - Dictionary Anywhere is an open-source and easy-to-use browser extension for Mozilla Firefox and Google Chrome that allows you to check definitions easily while browsing the web.
VerbAce - VerbAce-Pro is an easy-to-use Desktop Translation Software. Translate in a mouse click