Like most people here, I enjoy text editors and word processors that are streamlined and based around text, not document layout and production. A couple applications of note, historically:
Xywrite: a fabulously feature-rich-yet-streamlined text editor, designed largely for newspapers but usable by anyone. DOS-based, practically no mice allowed. I never used this much, as I'm typically a Mac/Unix guy, but I love the spirit of it, and noodled around (unsuccessfully) trying to get it to work on various ultraportable devices.
WriteNow: IMHO, the best lightweight word processor for Classic Mac. I composed probably an aggregate 400,000 words on WriteNow 4.0, starting with an Apple Duo 280, but then downgrading (for the long term) to a Powerbook 100. I would switch back and forth between WN4 and the American Heritage Dictionary Deluxe, and it was brilliant.
I still haven't found a favorite/ideal word processor for Mac OS X. (Xsphat: thanks for the referral to iText Express/Pro ... I just downloaded the demo.) The perfect word processor for me would be a full-screen (or -window) text editor with very basic text-formatting abilities (different fonts, different sizes, different styles (italic, underline, etc.)), the ability to export to standard file formats (RTF, DOC, XML, TXT), bookmarking (more on this in a bit), and performance/behavior that doesn't get in the way.
I tried (and abandoned) Mellel and Nisus Writer Express. Both were lacking in certain curious ways. E.g., Mellel's "full screen view" still has left and right margins. Oddly enough, and worst of all, both of these "light, modern" word processors would underperform with large documents.
This was the stupidest thing, and the computer scientist part of me is still appalled: I could work with a document of 200k words in WN4 on my PB100 (8MB RAM, 16MHz 68HC000 CPU) with no problems and lightning fast screen refreshes, even when paging/scrolling through the document. But load that same bulk-text document into NWE or Mellel on my G4 Powermac with 1 GB of RAM, and the screen couldn't even keep up with a leisurely 20wpm typing, especially at the end of a long paragraph. Stupid, stupid, stupid. Granted, these problems have been remedied a little with both applications, but I had to be their bug reporters in both cases, and the apps are still not as responsive as they should be. (Word, for all of its misgivings, has no problems with this.)
Today, I'm using Nisus Writer Pro, which is over-featured and still suspiciously underperforming. It's fine, but I'm not in love with it. It does, however, have one very nice feature: bookmarks. Like bookmarks in a PDF document, you click on a menu item and it takes you to a set location in your document. When you're working with a file that has many sections/chapters/parts and is 100k+ words in length, being able to zing around to certain parts without searching it out is one of those "never go back" features. Nisus's implementation of bookmarks is reasonable but not perfect. You still need to mouse over to a screen-cluttering menu, and you can't jump around by keystroke. I would probably be okay with NWP if the rest of the application was a better fit, but that's not the case and I'm still (passively) looking.
Oh, and I almost forgot: I use LaTeX frequently for work, via TeXshop and a fink-based LaTeX back-end. TeXshop is the best LaTeX front-end I've seen thus far for Mac OS X, although I admit I haven't shopped around much. I've considered using it for raw text processing, but I'd rather not work with (and around) typesetting primitives unless I have to.