A quick note--if you've read this blog before, you know that most of the editing I do is in Chicago style. In other words, it follows the rules of the Chicago Manual of Style, 16th ed. (That's what most major US publishers use as the basis of their house style guides.) Chicago style prefers Merriam-Webster when checking for preferred spellings and definitions, so that's the dictionary I turn to most frequently.
Of course, not everything is in that dictionary or that style manual. For example, I recently edited a legal thriller that borrowed certain conventions from legal writing. Those were nowhere in CMoS. And I routinely look up words that, it turns out, are nowhere to be found in Merriam-Webster. If what you're looking for can't be found, what do you do?
In those cases, I turned to special extra resources. I looked up a term in a legal dictionary to confirm that, as the author suggested, it should not be hyphenated. (He was right: "black letter law" would be hyphenated by CMoS rules, but we chose to leave it off thanks to the legal dictionary.) Other times, I'll look at regional dictionaries or the official website of a company, organization, or person, as was the case with Weezy down at #10 below. And yes, I've even checked spellings and definitions on UrbanDictionary--most recently for a book project with a teenage narrator.
Here's a roundup of what I've had to check on lately: