Mac: Popular iOS Twitter app (and our favourite) Tweetbot has released a public alpha version for Mac. Much like its mobile counterpart, the app feels like a supercharged version of the official Twitter client, adding a number of desired features while maintaining a simple and compact user interface.
One of the most notable features, as Matthew Braga points out, is trackpad gestures. Depending on how your multitouch trackpad is set up, you can swipe with your fingers to move from a tweet to conversation view. Tweetbot also makes it easy to switch between heaps of different views, show multiple timelines, easily view and upload photos, and customise the way it helps you tweet faster.
It's important to note that this version of Tweetbot is a public alpha release, meaning it's buggy and not feature-complete. It's still missing all sorts of things, there will be graphic glitches, and you're not going to get any support. Tweetbot's developers have noted that the app will cost money when it is finished, but you can use it during the development phase for free. For more information on the alpha release, check out the official announcement.
Tweetbot Alpha [Tapbots]