Microsoft Word.
I have a love/hate relationship with Microsoft Word. I'm been using the program - back in the day, "apps" were called "programs" - since version 1.05. That's pre-Windows even, the MS-DOS program, c. 1987. Word can be frustrating and fussy; it can show you one thing then print another. In particular, for complicated documents like novels, Word can skew the gutters, slant headers toward the wrong page edge, and kern fonts when you didn't ask it to (and fail to do it when you DO ask it too).
But at this late date, through all the many versions and "upgrades," I know ALL the bullshit with which Microsoft Word tries to trip you up. And I defeat it every time, and get out the document exactly what I want. There isn't anything I can't get to come out perfectly, be it a term paper, a union grievance, a screenplay, or a novel. And that level of expertise only came after years of writing and reams of wasted paper.
One funny thing Word has done from day one of offering grammar checking is berate you for using passive voice. (Take note, that sentence, as well as this one, was phrased in passive voice!) The grammar checker will single out every use, unless you tell it not to; I let it do so, since sometimes it's not a bad idea to avoid passive voice. Active voice is more direct and powerful, but doesn't accurately reflect how people talk; and passive voice is especially good if you want to leave the "object" of the sentence 'til the end, where it surprises the reader. This is handy for punchlines and reveals.
Despite my wars with Word, I enjoy when it offers suggestions. The one in this picture is a perfect example of why I leave grammar checker on. Hysterical stuff!
P.S. Please spare any comments suggesting that I switch to Pages or some free-ware Linux word processor. I left PC for Mac and Windows for Mountain Lion, but I know how to beat Word so I will continue to use it!