be Willing to be Intentional about Deliberate word choice

We had a discussion last night at group whether the word in someone's piece should be "willingly" or "willfully", and we never came to a conclusion on which was the right word. So I took some time to look up the words, and in the process rediscovered what I believe to be one of the greatest vocabulary tools out there.

I'm not sure what the official name of it is, but it's half-dictionary, half-thesaurus. Maybe it's best explained by the example I encountered:

Synonyms of "voluntary":
These adjectives mean being or resulting from one's own free will.

• Voluntary implies the operation of unforced choice: "Ignorance, when it is voluntary, is criminal" (Samuel Johnson).
• Intentional applies to something undertaken to further a plan or realize an aim: "I will abstain from all intentional wrongdoing and harm" (Hippocratic Oath).
• Deliberate stresses premeditation and full awareness of the character and consequences of one's acts: taking deliberate and decisive action.
• Willful implies deliberate, headstrong persistence in a self-determined course of action: a willful waste of time.
• Willing suggests ready or cheerful acquiescence in the proposals or requirements of another: "The first requisite of a good citizen ... is that he shall be able and willing to pull his weight" (Theodore Roosevelt).

From: The American Heritage® Dictionary of the English Language, Fourth Edition
Copyright © 2006 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
Through: Dictionary.com

I love getting a list of synonyms in this format, because it illuminates the nuances of each word, and enables me to choose the perfect word for what I'm trying to convey. So, please cheerfully accept [be willing] my proposal and act to achieve your aim [be intentional] of creating a masterpiece by performing premeditated [deliberate] research into your choice of words.

So, what's your favorite tool or resource to turn to while working to improve your writing?

P.S.: I've changed my mind and now believe "willingly" is the better choice.