LEO (http://dict.leo.org/) is a great online dictionary for German-English, German-French and German-Spanish.
I made a extension for the deskbar-applet for searching the LEO dictionary.

You can have it too: download this tar archive and extract it. (sorry, had to append .png for uploading)
Install for all users (requires root privileges):
- Put the png-file in your pixmaps (probably /usr/share/pixmaps/).
- Put the python-file in /usr/lib/deskbar-applet/modules-2.20-compatible/
- All logged in users need to restart the deskbar-applet (e.g. killall deskbar-applet), or remove from panel and add again
- Each user that wants to use it has to enable it in the deskbar preferences.
Install for you only:
- Throw away the png file, you can't install it anyway (afaik)
- Put the python-file in ~/.gnome2/deskbar-applet/modules-2.20-compatible/
- Restart the deskbar-applet (e.g. killall deskbar-applet), or remove from panel and add again.
- Enable it in the deskbar preferences.
How to use:
- Write "en-de Grammatik" or "en-de diddly" or "fr-de Grammatik" or "es-de Grammatik" in the search bar.
Cool features:
- Caching: The folder /tmp/deskbar-applet/ is created with chmod 0770. Searches are cached there (shared between users).
- When clicked, text is copied to clipboard
- Encoding is handled correctly (from iso-8859-15 to utf8)
- The priority is exactly as the matching quality on the website (direct matches, then in words, etc.)