If you've ever felt the dictionary or thesaurus in popular office applications doesn't go far enough, consider installing a copy of WordWeb instead.
This little utility sits patiently in your SystemTray, waiting for you to enter a word. It then offers a definition - along with synonyms and even types of, or parts of - the word you've entered.
For example, entering 'tree' will bring up the following definition: "Tall perennial woody plant having a main trunk and branches forming a distinct elevated crown". But press on the Types tab and you're presented with a list of every type of tree you've ever heard of and even many you haven't.
It's also easy and great fun to click on synonyms and find yourself effectively surfing a thesaurus.
WordWeb is a dab hand at solving anagrams and filling in the blanks as well, but this feature is sadly not available in the free version.