Before you check out the video screencast, just think of a dictionary or thesaurus that you’d like to have added to your Mac’s Dictionary, and then Google around to hopefully find a free or open-source version of a Leopard-compatible dictionary. It would need to be a folder with the file-name “.dictionary”. Note that this manual method works only with Leopard (10.5) and Snow Leopard (10.6). Or the third-party dictionary might come as an installer package (”.mpkg” file), which helpfully puts the new dictionary in the right place within ‘Dictionary’.

Once you’ve got the dictionary you want downloaded (see below for some links to get you started), you’re ready to see the screencast on how to add it, and get the very most from Dictionary.app on your Mac:

Additionally, here are some useful links (to free resources) to get you started in your search for an extra dictionary:

A German-to-English dictionary in the form of a plug-in, based on dict.cc site’s dictionary.
A medical terminology dictionary in the form of a “.dictionary” folder.
The OpenThesaurus German Dictionary plugin.
The BeoLingus German-to-English dictionary plugin
The CEDICT Chinese-to-English dictionary which is based on the open-source CEDICT project, which is occasionally updated. This is version 1.2.
The very large English-to-Chinese dictionary pack which contains four dictionaries (three of which are Traditional Chinese; one of which uses ‘Simplified’ characters).
An English-to-Arabic dictionary plugin.

There are more free and shareware dictionaries for Dictionary.app online.