Syrinx is a free, customizable Twitter client that gives you maximum control over how you tweet.
Although Syrinx isn't as aesthetically pleasing as Tweetie, it is free to use (and doesn't feature ads unlike the free version of Tweetie) and gives you considerably more control over how how you customize the client. For example, you can customize background colors or change the look of Syrinx completely using the built-in themes. There's also a handy bookmarking system which aims to help you keep track of unread tweets.
Syrinx also includes Growl support although this isn't really necessary as the Dock icon has a red number on it when you have a new tweet to read. The latest version has introduced useful new features to search all of twitter as well as your own tweets as well as retweeting, tweet deletion, URL shortening and much more.
At first, you may find it a bit unclear how you actually write your own tweets but you just have to start typing in the empty box at the bottom of the client. However, I miss the ability to select how I want to shorten URLs like Tweetie gives you and the overall interface feels somehow more cluttered and confusing than the latter.
Overall though, for a free Twitter client, you really can't complain too much about Syrinx.
