TweetDesk
This is a gorgeous way to display a clock and tweets together. You can either choose to display your timeline in your account or you can choose from a list of twitter users to follow.
Boxcar
This is not really a twitter-only app, this allows you to get push notifications from facebook/twitter/email that you want. For the twitter service, you can set your favorite twitter app to launch to view the latest tweet. (update) now offers all services free with the showing of ads.
PictTweet
If you like doodling and sharing them you might want to try this. there is a paid version where you can draw on photos you take on your iPhone. (didnt test this)
TweetShow
This is similar to the Tweetdeck above, but it doesn’t display a clock. You can choose to watch trending topics as well. (didnt test this)
TweetMusic
lets you tweet about what music you are listening to and lets you include a link to the itunes store or youtube video about it. It has a Lite version that is limited to 2 posts a day. (didnt test this)
TwittrTapper
Provides default messages so that you can tweet quickly if you need to leave a generic message on twitter. (didnt test this)
TweetMic Lite
Allows you to post recorded messages on twitter. The pro version allows you to add a photo or an interactive map as well. (didn’t test this)
Twickret Free
allows you to send encoded messages so that only a specific number of people will be able to read your message using the codeword. (didn’t test this)
