OS X comes with a list of all the words in Webster’s 2nd International Dictionary (234,936 of them to be precise). These words can be found in the file /usr/share/dict/words. I find it useful to search this list if I’m not sure how spell a given word by using the following command:
% more /usr/share/dict/words | grep [pattern] | more
This should return a list of words from the dictionary that contain your [pattern] (don’t type the square brackets) of interest.
Another interesting file is /usr/share/dict/propernames, which is a list of common (mostly Western) names (useful if you’re having difficulty coming up with a name for your newborn).
ZIP Codes, Area Codes et al