Mac OS X's built-in spell checking abilities are fantastic, but what if you need to edit the custom list of words you've been building, or you want to nail a few birds with one stone by adding a collection of words in one fell swoop?
Christopher Breen at Macworld has a good tip for just such a case. It turns out that once you begin adding custom words to your dictionary (by right-clicking them in most Cocoa apps and choosing 'Learn Spelling'), a folder aptly named Spelling appears in your Home user folder. In there is a file that I believe is named after your particular language; in my case - English.
Opening this file in something like TextWrangler (or possibly OmniOutliner?) will allow you to edit the dictionary, including adding your own words a little more efficiently. Check out Macworld's tip for the details on how the rest of this works, including a basic method for (sort of) synchronizing your dictionary between Macs.