Twizzle is a free, beautiful, twitter client for the iPhone. It's stock full of features but will never get in your way.

Features:

• Multiple account support
• Landscape tweeting
• Twitpic intergration to upload pictures to Twitter on the go
• View and favorite tweets
• Retweet, @reply, and direct message anyone Twitter allows you to
• View anyone's friends/follower count, description, location, recent favorites, tweets, and mentions
• Click links directly in tweets to access them in Safari
• Follow your mentions and direct messages