Learner's Dictionary Online
A good editor or copy desk should have at least one learner's dictionary available. Merriam-Webster has just put its online at learnersdictionary.com
Such dictionaries are written for English as second language and other English-learning students. Some might find it beneath them to consult one ("after all, English is my mother tongue"), but learner's dictionaries bring a valuable dimension to your resource shelf.
Unlike the dictionaries we traditionally reach for, learner's dictionaries provide more guidance on idiom and collocation (words used in common patterns).
For instance, look at the entry for resource in M-W's Learner's Dictionary. The usage examples show that "for" is the common collocated preposition used with "resource." Of course, you say, I knew that. But these kinds of things do come up on a desk (I chose this word because a recent Copyeditor newsletter tip of the week included a reader's question of whether a charitable organization was a "resource to" or "resource for" abused women).
Now look at the entry in M-W's standard dictionary. It gives you much less on idiom, but much more on etymology and synonyms.
(Another example: Rein in the M-W standard and in the learner's, which has many more phrasal uses. I just wish M-W would show misuses under their improper spellings, such as "free reign," and then point to the correct entry.)
Put the two books (or online sites) together, and you have a pretty good resource for those sticky issues.