WordWeb is a really neat, free program for Windows. It adds a pop-up dictionary and thesaurus to just about any application, and it's both quick and easy to use. It's brilliant if you're writing any form of document, and especially good to help the kids with their homework too. It can be used to look up words from almost any program, showing definitions, synonyms and related words. It includes pronunciations and usage examples, and has helpful spelling and sounds-like links.
Download Link: http://www.wordweb.info/