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:
TweetDeck is a feature-stuffed multi-column Twitter app for the iPhone. It is already a major desktop twitter client, running on Adobe AIR, with all the features anyone could ask for (including multiple accounts as of 0.26), so it seemed like they would spend all their time making a great user interface. They packed it all in there, but at the expense of usability. From their website:
TweetDeck is an Adobe AIR Twitter app for power users. Using a column layout, TweetDeck allows a user to set up many feeds at once. Some are specialized to Twitter, such as your friends, @ mentions, direct messages and so forth. But the real power lies in their numerous advanced search features, which filter the public timeline any way you desire: