[zapu]

It's not the first one I've seen, but the general idea is that it lets you write in a completely distraction free environment (when you full-screen it).

[wanda]

WriteRoom (http://www.hogbaysoftware.com/products/writeroom)

DarkRoom (http://jjafuller.com/dark-room)

iA Writer

Write! (http://wri.tt)

To name but a few of these distraction-free writers.

Write! is cool but Windows only. iA Writer isn't free but Dropbox integration and mobile version justify the price for me.