Dictionaries do more than offer writers and editors spelling advice. They make explicit shades of meaning, offer grammar and usage advice, list word histories, and more. For consistency, you should choose one dictionary to be your main dictionary, using other dictionaries to debate questions.
The American Heritage Dictionary
This is the latest edition of AHD, released in fall 2011. The usage notes alone are worth the price of admission.
Merriam-Webster’s Collegiate Dictionary
The dictionary of choice for The Chicago Manual of Style, MWC is a well-respected dictionary.
Webster’s New World College Dictionary
WMW has a new lease on life with new owner Houghton Mifflin and is still the go-to dictionary journalists.
Concise Oxford English Dictionary
Another recently updated dictionary, COD is a good backup dictionary for American writers and editors.
There are so many dictionaries online that using a dictionary is incredibly easy. My collection of bookmarks on Delicious has links to some of my favorite online dictionaries.