What references do you use?

I have a lot of references at my disposal that I use on a regular basis (grammar books, etc.) but my two go-to, must-have, use-them-thirty-times-a-day sources are The Chicago Manual of Style and Merriam-Webster's Collegiate Dictionary (as well as their Unabridged, online free, French, Spanish, and Medical dictionaries). For UK & Australian clients who choose to maintain UK spellings and style, I use New Oxford Style Manual and Oxford Dictionaries. For a more in-depth answer, check out my article Tools of the Trade.