I wrote a plug-in for the Mac OS X 10.5 Dictionary that provides the complete German-English dictionary from the well-known dict.cc site for offline use. You can read more about the plug-in -- and download it -- in this blog entry on my site.
The plug-in provides access to the German-American dictionary in Dictionary itself, and other core OS X services like Spotlight and the Dictionary widget. You can use this to help translate web pages, etc. The tool itself is open source, but the vocabulary data itself is not, as explained in dict.cc's Terms of Use.
To use the plug-in, run the installer package, then launch Dictionary and open its preferences. Check the box for the dict.cc dictionary, and you're good to go.
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080318071042138