Marcin Wichary made a typewriter simulator. It's not merely another focus-writer type web app with a blotchy monospaced font and a clacking sound effect–though it certainly has those things!
It ended up being more complex than it seemed. You can overtype, rotate paper, set margins, use Wite-Out, change ribbons… Let me know what you make!
All play and no dull makes play a jack boy. Be sure to click around on the left for some teasers from Wichary's book, Shift Happens, on the history of keyboards. I'm looking forward to it more than anything else this year.