Personally, I use CitySearch, chowhound, and zagats, in that order. Since they have already been mentioned in posts X, Y, and Z, however, I probably should find a new source.

Of course, really, my first stop is usually at the front desk of the hotel where I try to describe to the desk clerk what I'm looking for and ask them where they personally would go. Lots of places have menu books behind the desk (particularly, I find, at mid-sized hotels without restaurants in small/midsize towns). In larger towns, I've heard the suggestion to ask your cabbie where he or she would want to go for the very best (describe what you are lookiing for) if time/money were no object if you want great suggestions from folks who hear it all.