Honestly, I expect twitter applications to be among the first apps that people start developing once the N900 ships.

The Twitter API is really simple to use (everything is done through HTTP) and it presents a good starting project.

I'm hoping to see a really good "widget" for Twitter, as that's the first thing I'm working on (and nothing would make me happier than to have somebody else make what I want so I don't have to do it myself).