Pages and the word processor included in NeoOffice. My problem with Pages is that it's not really 'corporate' enough for my requirements. Pages is great for someone working in SOHO or maybe NGO type organizations, but the enterprise is just too entrenched with Office compatible data that I can't abandon it just because it's made by Microsoft. Nissuswriter also has great reviews. Might wanna check it out.
I've been using a demo version of Ulysses for my occasional writing needs. It's probably something that people like Butch Dalisay should look into ... paging Butch! I love the amber on black fullscreen option. Takes me back to the sleepless nights of writing my thesis on a CGA monitor and 386DX PC! Download here.
Haven't personally used it much but Writely looks promising. Its a web word processor with a neat collaborate feature. This is something Microsoft might be worried about with regards to their Office Suite, as this might be the start of the thin clients no-need-to-install kind of application. Do give it a try and let us know what you think!