jDictionary is licensed under the LGPL license, so you can download and use this program for free, and download its source code too.
jDictionary is plugin based and has an easy to use plugin management system which is able to download and install the desired plugins automagically from the web. Installing or upgrading one or more plugins (even simultaneously) is just one click!
It's available for windows and linux.
Learn more about jDictionary.