Write Or Die is a web window that will time and punish you while it obstreperously counts your words; WriteRoom takes over your desktop, basically disabling the myriad distractions that can get to you when you're trying to writing using a standard word processor.
WriteRoom looks more or less like this; little green type over a black background:
I used WriteRoom a bit this week to do some prewriting on a new chapter I'm writing, and kinda liked it ... for a while. I had some trouble bumping up the type size, and the red-squigglies were just too red in front of the black background ... but I like the idea. These apps try to return us to writing from the land of multitasking; they try to remind us that PCs were once just glorified typewriters.