I'm one of those people who has trouble writing at length on my main machine, because of all the distractions it offers. Email and messaging and social networking: they all combine to form the "ludic loop" that Mark recently blogged about.
I've tried various things over the years to help keep me focused, from simple full-screen word processors such as WriteRoom and FocusWriter to gadgets like the Alphasmart and Freewrite. But apps are a tab away from fun, and glorified typewriters tend to expose their limitations in odd and frustrating ways.
After a lot of experimentation, I've arrived at a best-of-both-worlds option: proper apps running on a tiny old iMac from when Apple switched to Intel chips. It's modern enough to run good software, play music and hook up to useful services like Dropbox, but so old (and tiny) that there's not much else you can do on it except work. And it's immobile, too, so it creates a space just for that one task, which I think helps.
Even web browsing is just right: the older OS X 10.6-compatible version of Firefox it runs will access research resources well enough, but the media load on dangerously interesting sites (including Twitter and Facebook) renders them almost unusable.
There are dangers to this approach. If I were cunning I'm sure I could rebuild the ludic loop on this, by hitting the mobile versions of websites and exploring what other apps work on Snow Leopard. But its age (and adorable low-res 17" display) are so far dissuading me from trying. I'm writing more.
People keep asking about this keyboard! It's the Planck ($100 on Massdrop). I made the wrist wrest myself out of a plank, to go with those old KLH Model 19 speakers. I'm not entirely convinced the Planck is for me, but fans swear by them. I'll probably switch to something just as compact but a little more traditional, such as the MiniVan.