Writer - Dark Room / WriteRoom for the web
And it was a very cool idea, but there was nothing similar for Windows. Until Dark Room came along. Dark Room basically emulates WriteRoom's functionality on Windows.
So what's missing? A web-based version, of course.
Now, normally I'd say that doing a web-based version of a text editor when very strong versions offering identical functionality already exist on both Windows and Mac would not be worthwhile. But as much as Writer, the web based version, is simply replicated the functionality of these other applications, it still stands on its own merits.
Two things about it make it worth having in your bookmarks. The first is that you can use it anywhere. No memory key to carry around, no worrying about what OS you're using. It just works, wherever you are. The second is that if you create an account, all of your documents also follow you around.
So if you like the look and feel of Dark Room or WriteRoom, check out Writer.