LingoPad was clearly designed for German students who are studying English, but it also works well for English speakers.
The program comes with a useful German-English dictionary and a Whales and Dolphins dictionary that seems to be more of a novelty. You can quickly switch between dictionaries; when in a dictionary, just click a button to shift the focus from German to English and back. The interface is in English, but the dialogs are in German, oddly enough.
Many other dictionaries are available for download, but only one other may be useful to English speakers: Arabic-English. There is a Chinese-English dictionary, but it uses a character set that isn't included with the dictionary. Other languages include Spanish, French, Japanese, Kurdish, Norwegian, Turkish, and more. Note that the usage instructions available on the Web site are incorrect: To download a new dictionary, go to the Dictionaries menu and select Manage Dictionaries to access the Dictionary Update screen. Also note that the developers do not specify which versions of Microsoft Windows this software runs best on. I tested it on Windows XP, however, and it worked just fine.
Those quibbles aside, the German-English dictionary would be very useful to native speakers of either language. The dictionary seems comprehensive and well written, and the LingoPad interface is easy to use. German speakers studying English should visit the Lingo4you Web site, which provides many useful resources free of charge.
LingoPad is free, but the developers accept donations via PayPal. Note that the download link takes you to the developers' Web site.