I've been playing with www.twitter.com quite a lot over the last week or two.
On the send side, I used the tweetsharp project to send tweets - if you're a www.runsaturday.com user then you'll have seen this - if not, then you can't see it! - http://code.google.com/p/tweetsharp/
On the browse side, I used twitter.js to show people's tweets on their runsaturday profile - see http://www.runsaturday.com/Profile/TabId/82/UserId/3/Default.aspx for an example - http://remysharp.com/2007/05/18/add-twitter-to-your-blog-step-by-step/
Both libraries come recommended