Dictionary is an application included in Mac OS X. It is a standalone program. You can also access it as a Widget app. Its dictionaries are used by the system as a spell checking resource.
Application interface is very easy-to-use and consists of a window with a search field. The results show up as you type a word. Partial word search is also supported. A word article has a word's pronunciation, synonyms, examples of phrases that word is used in, and the word origins.
The dictionary can also be accessed through a contextual menu of a selected word in applications that use Cocoa APIs(application programming interfaces). Right click on a selected word should bring up its definition and/or synonyms.
The benefits of the program include: a user ability to switch between languages on the fly (this is helpful in cases when you are dealing with British and American folks at the same time), program integration with Wikipedia, and program ability to include more dictionaries (however, you need to use another application to do so).
Since the program is tightly integrated into the system, it is used by every single user in one form or another. Professionals can be sure they've spelled a word correctly, while students can find better synonyms when writing a paper.
In summary, the program is free and it has over 200 000 words in its dictionaries. Also, program ability to include more dictionaries exclude the need to buy expensive third-party software. The disadvantage of the program is that it doesn't support syntax error checking but I hope it will be included in the later versions of the Dictionary.