Twitterific is a great little Macintosh program that lets you get (and make) your twitter updates without having to have the browser window open.
There does seem to be a couple of similar Window/PC programs that lets you do the same thing – Twitterlicious is one, and Twitbox is another. I’ve never used either, so I don’t know if they’re good or not.
Twittervision is a google map that lets you see where the latest twitter posts are from, all over the world. This morning when I got up, people were twittering in Hong Kong.