For German users and English-speaking users with German-language needs, Philipp Brauner has made available the dict.cc Dictionary Plugin for Leopard, which adds the dict.cc vocabulary compilation to the OS X Dictionary. dict.cc is a community generated and verified English-German dictionary. The large download size (125M) speaks to the expansiveness of this compendium.
The plugin not only enhances Dictionary.app, but other services that tie into the OS X Dictionary, including Spotlight and the Dictionary widget. It comes with a handy installer which makes the plugin easy to install for all users.
The plugin borrows from a previous project by Wolfgang Reszel which integrated the BeoLingus vocabulary in the same manner. The dict.cc plugin is open source and the dictionary data is licensed separately, but available for use. Whether you're an English speaker with a desire to dabble in Deutsch or German is your native tongue, it's worth the sizable download.