Thanks so much, you guys!
I have tried Word Web. For me, the thesaurus isn't rich enough, and it doesn't integrate well with Word.
What I really don't like about Word's thesaurus is how slim it is. There are very few synonyms, and they often aren't the best choices.
I have a fantastic thesaurus (hardcover book)--Roget's International Thesaurus, 5th edition, 1992. If I look up a word, say, "peaceful", I then get 7 choices of words that are more specific that can mean peaceful (calm, comfortable, quiescent, homelike, in accord, pacific, moderate), and for each one of those words I can look up the synonyms--and there are at least 20 synonyms for each word, usually more. Those choices are rich, and help me find words that aren't too overused (like "beautiful" is), and that also help me say what I want to say more precisely or specifically or just...better.
I so very much wish I could find that in software. When I'm writing or editing within Word I don't always want to turn to my hardcover thesaurus--it feels so much easier to turn to software on my laptop/desktop. But...to not have good choices doesn't help me.