Thanks to a thread of Maud Newton's I have discovered what may be the Holy Grail of writing these days: A way to do it without checking email every five minutes and losing half an hour on Facebook.
For Windows users there's DarkRoom, (and WriteMonkey -- thanks to commenter Josips, I now prefer WriteMonkey) and for Mac-heads there's WriteRoom. The downloads are free, or at least DarkRoom was.
They do nothing but clear your screen of everything except your text -- they turn your computer into basically a typewriter. I'm pretty sure I've at least doubled the amount of time I write at a stretch using DarkRoom. For example, an hour and a half just went by as I wrote my novel, and it never even occurred to me to dick around online, because I could see nothing onscreen but my novel, which kept drawing me in, in, in.