When WriteRoom debuted, many of us were impressed with the software and the idea of a focused interface that just lets you write. WriteRoom allows you to write on a screen of nothing but text, and the default view mimics the way many of us used with our first word processor (myself included): green text on a black screen. Still, if all you want is a screen of text with very little interface, here's a free and quick solution.
Start with Backdrop, a simple application for obfuscating your desktop or other applications. You can get fancy and include something like Spirited Away for auto-hiding applications you aren't using, but that's not the point here (and I don't think that app works in Leopard). I am assuming you're settling in to "just write." I find the default gray works fine, but you can customize the color and make it black if you are ready to zone out on your text.
Next, fire up TextEdit. While you can zoom the TextEdit window, I find the wide view of your text is distracting. WriteRoom is nice because you have big margins on a wider screen, making it easier to read what you write. To simulate this, just resize your text edit window to mimic a page of paper in front of you. Go to Preferences in TextEdit and alter the default font settings (for rich or plain text, depending on what you prefer). You can also change the default window size, or text area. I found a height of 45 with rulers turned off went from the top of the screen to the bottom on my 15" MacBook Pro. I use 18-pt Helvetica, but if you prefer Marker Felt, go for it.
That's it! You can't customize TextEdit as much as you can Terminal, but if you want a universal text tool that can serve as a distraction-free environment, this works in a pinch. You could just as easily pare down your browser view and pop Google or Zoho Docs over Backdrop -- or for collaborative editing, open a free Etherpad page. If you do a lot of writing, you may want to look into WriteRoom. There's also an iPhone app for WriteRoom, which includes a mechanism for editing docs on your Mac (sort of).