iTweet (http://itweet.net) is a web-based twitter client that supports everything that the original Twitter site needs to.  Created by the folks at http://illusionfactory.com/, it has everything that a Twitter app like Twhirl has but it is a web app so it can be used on any computer with no download necessary.  It uses your Twitter username and password so no account setup is needed.  It also has many of the features that Twitter users have been asking for including retweet and autorefresh.  It also has all the features of the regular Twitter site even an import feature for your Twitter theme and background.  The only problem that I have with iTweet is that there is no back button to go to the last page.  This is needed because iTweet uses animation that keep all activities on the same page.  Other then this small problem, iTweet is perfect and has become my web interface of choice for Twitter.  So, check it out at http://itweet.net today.