TwittMS for iPhone

From Pleasant Software for the People:

Open TwittMS, tap on one of your buddies and type in your message. TwittMS keeps track of your conversations in chat-style views, so it is easy to pick up a conversation where you left off. View your conversations as ongoing chats, group your favorite contacts on multiple pages, browse all of your followers and add shortcuts easily, optional landscape keyboard, optimized for responsiveness and low bandwidth networking, and exquisite user interface and animations.

Note: The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.