Tweetree is an alternative to the web interface at Twitter.com featuring expanded URLs and threaded conversations. Aside from those features, it sports a web interface whose primary difference from twitter.com is skin deep.
Tweetree handles all of the common Twitter streams & features like home, @ mentions, direct messages, and favorites. It has basic search and lets you save your common searches. It has a handy button for retweeting as well.
Echofon for iPhone, formerly TwitterFon, is a super-fast, easy-to-use Twitter app sporting video recording. They changed their name presumably to remove the whole word Twitter out of the app name to avoid trademark trouble. Although the free version is definitely stripped down compared to their $4.99 Pro version, Echofon for iPhone has quite a few features:
Tweetie is a gorgeous, full-featured desktop Twitter client for Macintosh. atebits is the company behind Tweetie, already known for the fantastic iPhone version of Tweetie. Notable features include:
Twitterrific for iPhone is a fantastic, feature-packed iPhone twitter app for beginning and advanced users alike. First of all, their 2.0 release saw a major performance and feature overhaul compared to the original version, which was a great twitter app to begin with. This interface is amazing! It's quick and responsive, and scrolling past a hundred tweets with one flick is no problem. But it's not just looks, because there is a LOT packed under the hood:
Twitter Analyzer is a web app that provides granular twitter analytics for any user. They bill themselves as "Twitter Analyzer Is The Most Advanced Twitter Analytic System In The World." It has a vast selection of options for viewing your twitter data. It is simple to get started; you enter your username. It thinks for a while, then loads daily graphs of your data filtered many ways: