Lingoes is a free dictionary ( desktop application ) and It is the best dictionary you can find. Calling it just a dictionary would be wrong actually because it includes :

Search as you type in feature for word meaning with 80 language support
Embedded speech engine.
14 Language Translation engines.
Supports inline find i.e. Select any word in [...]

Ninjawords is an online dictionary, open the site and simply key in your word to get the meaning. The definitions come from the fast-growing Wiktionary. If Wiktionary does not have the definition, they use Princeton WordNet dictionary.
The beauty of Ninjawords is:

It is quite fast and hardly takes any time to display the meaning of [...]

Wordsmith is an online service which allows you to find a definition or meaning of a word by just sending them an email. All you need to do is mail to wsmith@wordsmith.org with subject as define yourword . You need to edit the body part of the mal. Just keep it blank.

Moreover it [...]