MacDING is an English German dictionary-lookup program for Mac OS X. It is a reimplementation of Frank Richter's DING (DIctionary Nice Grep) lookup program for Unix/Linux Systems (using Tcl/Tk).
MacDING is natively implemented in Cocoa and provides therefore an eye-candy Apple interface. The functionability at this point however, is not as comprehensive as the one provided by the original software (e.g. no ispell/dict lookups).
In case you need a more comprehensive application, you might consider installing the original software which runs under Apple's X11.
MacDING will also provide German Norwegian and German Spanish tranlsation with the help of custom dictionary files.