It seems every writer, coder or GTD aficionado has a favorite plain text editor on the Mac these days. For everyone who starts each day with a screenful of untitled TextEdit windows full of different short notes (that would describe my dear spouse, for the record) there's a TextMate guru, an Elements fan, a Scrivener loyalist. It's a golden age for the .txt file.
One of the strongest contenders in the "distraction free writing" category is Hog Bay Software's WriteRoom, which comes in both Mac and iOS flavors. The Mac version saw an update to version 3.0 on Halloween, adding a NaNoWriMo-essential feature: dynamic word counts. Each file now shows a running count, and with a new session tracking engine you can easily track your output to a .csv spreadsheet file.
The new build also adds themed display options, better handling of long documents and Lion-friendly Versions and full screen mode support, in addition to the "classic" blackout approach that hides other active apps. You might not need the full anti-ADHD power of a single-window UI for your writing, so WriteRoom works just fine with traditional document windows.
WriteRoom for Mac is $9.99 whether you pick it up in the Mac App Store or directly from the Hog Bay site. Those who bought it within the last year are entitled to a free upgrade; older licenses (including those received in a previous MacHeist software bundle) are entitled to a half-off discount; you can upgrade for $4.99. Note that WriteRoom 3.0 is 10.7-only, so if you're still on Snow Leopard you'll want to stick with the older versions.
The iOS version of WriteRoom, which of course cooperates nicely with its Mac cousin via Dropbox sync, is available on the iOS App Store for $4.99; it's a superset of Hog Bay's ad-supported PlainText app, with more visual control and other advanced features (disabling autocorrect, extended keyboards, etc.). One compatibility note: the iOS version does not edit .rtf rich text files, but the Mac version can. If you intend to work on your magnum opus from your iPad, stick with basic plain text files.
For myself, I haven't quite found the perfect iPad writing app yet. I love Elements' scratchpad for side notes, while I crave the research power, inboard browser and speedy Markdown formatting of Writing Kit -- but the unchangeable paper background gives me itchy eyes. WriteRoom iOS is among the most comfortable and aesthetically pleasant editors I've tried, though, and for narrative work without a lot of links or Markdown syntax, it's a winner.