Although I said for GNU/Linux in the title, but this can be used on MS Windows as well. The interface is written using Qt 4 and I gotta say, it looks really good. With GoldenDict you can use Wikipedia (known for painfully long, boring articles :P) or any other online websites that supports the MediaWiki framework for online lookups if you like. It supports various languages + translations as well. A pretty powerful app.
But if you prefer it to behave as an offline dictionary, then you'll have to manually download a dictionary for that (there are both paid and completely free dictionaries... both are excellent according to users). If you use Ubuntu 11.04/10.10/10.04, etc then you can install GoldenDict by entering the below command in your Terminal.
sudo apt-get install goldendict
Then, as said before you have to install a dictionary afterward. You can find all the proper instructions on this GoldenDict dictionary page concerning that.