Tweetbot is by far my favorite Twitter client for iOS, and today they just released a free alpha of their Mac client. (the download link is still pretty busy as of this writing, so try this download as an alternative.)
Already first impressions are good. Its UI has very similar functionality and layout to the iPad version. It still has many rough edges (e.g. thin font ligatures, stuttered scrolling) but the app is off to a good start.
Given the quality of Tapbots other software I’ve got high hopes for Tweetbot. Right now Osfoora is my main Mac client, but I could easily see myself switching to this.