I want to like NetLingo. The definitions are good and easy to find. However, they do have "definitions" of Internet Jargon and Slang so kids can look up what WTF means in class. Sure the words are blanked out, but I am afraid some students may be more interested in what IYSS stands for than their assigned terms.
This site is good for younger elementary students. This site is very simple. The words are listed in alphabetical order and are very easy to find and define.
If all else fails, you can always resort to some of the tried and true originals.