|Mac's Dictionary app|
What makes Mac's Dictionary so vital to my writing is the combination of resources it puts together. There's a Dictionary, a Thesaurus, and Wikipedia. The preferences for Dictionary allow you to extend your options to a British Dictionary and Thesaurus, as well as Japanese.
One of my worst writing habits is to unconsciously get into ruts. When a sentence isn't working I'll play with the wording in my head, trying to find and fit all of the right words together so they work exactly the way I want them to. Many times I'll re-read those sections later to find that I've reused a key word from another sentence earlier in the same paragraph.
This is why I've set my Dictionary preferences to show the Thesaurus first. Being a big word fanatic I tend to get lost in Thesaurus, clicking from one hyperlinked word to the next, looking for just the right word for the sentence in question. The ease with which Dictionary allows this to happen makes thumbing through my old hardbound copies seem downright antiquated.
In my work-a-day job I use both Macs and PCs. All of my writing, though, is done on a Mac. The PC side has something similar called WordWeb. WordWeb has a dictionary and thesaurus combined. There's a free and pay version available.
Are you a Dictionary fan? What's your preferred dictionary/thesaurus application? Is there one you prefer on the PC side? On the Mac side?