Nobody ever wants to add anything to the dictionary, do I look like a dictionary? Do I know better than the collective knowledge of humanity? Yes I understand there are reason to add stuff to the dictionary, but it's not something that you do regularly, and there's no reason for me to not have to go into a preference window to set something like this up. Keep Your Word lets you build a custom dictionary that lets you classify and group words how you want to. It costs $24.95 and a 30-day trial version is available for download.
Bambooapps has announced the release of Keep Your Word 1.0, a custom dictionary builder for Mac OS X.