Word Web Pro is a program I’ve used for as long as I can remember. It’s a combined dictionary and thesaurus and sits in my system tray, ready for any query I might throw at it. However, it’s real strength is its ability, when I highlight a word and click the system tray icon, to pop up a definition and a list of synonyms. For a writer, that’s invaluable. The basic version is free. I have the Pro Bundle version and for A$129 I get audio pronunciations, many other dictionaries (e.g. Oxford, Cambridge), and geographical names.