"What is the name of the ending of a letter, e.g. "sincerely" or "yours truly?" I guess it would be the antonym of "salutation?""
Complimentary close. Occasionally just "close" or the bureaucratic solecism "complimentary closing". (The stock use of "sincerely" as the complimentary close in almost all American business and government correspondence must be one of the greatest abuses of any word in modern English.)
"I am writing a book and have a lot of questions like this (maybe 10 or so). Does anyone have ... with antonyms (online, preferably)? Is this newsgroup the one I want to be posting questions like this to? Thanks, Brad"
There's the OneLook Reverse Dictionary online,
http://www.onelook.com/reverse-dictionary.shtml
Bernstein's Reverse Dictionary and the Oxford Reverse Dictionary are available from the usual sources. The visually inclined and non-native English speakers may find the Reader's Digest Illustrated Reverse Dictionary useful.
Chris Green