WriteRoom is an extremely handy application that lets you do one thing; write, without distraction. It does so by blanking out the rest of your Mac, letting you concentrate on putting your thoughts to bits and bytes.

There is a free version and a paid version for $24.95 USD which offers a few extra features.  I got my copy through one of the MacHeist deals.

WriteRoom defaults to a black screen and green text – very old school – but you can configure the text & background colour, font, and a host of other options.