In response to Bill's question about what actual BOOKS I keep for reference, I'm forced to admit that there aren't any. The only exception to this is the comics that I keep on hand for story and visual reference for the stories I'm working on, but I'm pretty sure that this isn't what Bill had in mind.
In lieu of that, however, I can list a few online resources that I consider absolutely critical:
Wikipedia: I can't express how much Wikipedia has raised the bar in terms of what it means to be knowledgeable. It's not enough to just know the basic facts -- you have to go deeper. In any subject on which you discourse, you must be at least as informed as wikipedia, or you're an idiot.
reference.com: Dictionary and thesaurus all in one. Having been caught out once or twice misusing or misspelling words in my printed work, I've gotten a lot more careful about such things, so I never have to hear from another reader, "That word you keep using -- I don't think it means what you think it means." For years, I thought "matriculate" was a synonym for "graduate." It's not.
Project Gutenberg: Original sources, especially classics, galore. When you need to go back to the source material, this is the first place to start. I spent quite some time on here trying to verify that Heraclitus actually wrote, "Only change endures." As far as I can tell, he didn't. This, apparently, is one of those things that has filtered into the popular consciousness without proper citation, like "Play it again, Sam" and "Elementary, my dear Watson."
Babelfish: I don't know about you, but I often find myself needing a phrase or two from a foreign language. The trick is that you can't take babelfish's word for it. You need to spend some time googling the result to make sure that this is something that people actually say.
These are some of the obvious ones. What are some of the less-obvious?