Tweetdeck was one of the best alternative Twitter clients for Windows, Mac, Android and iPhone. It was full of features, had a minimal design and had the ability to also connect to your Facebook or Foursquare account. However Tweetdeck was bought by Twitter because it realised the popular alternative was outpacing Twitter itself, having such a huge market share in the way people accessed Twitter. It was a move which Twitter just had to do to keep up with the same.
Twitter then replaced the desktop version of Tweetdeck with a crappy version of their own. They removed many of the favoured features which people used Tweetdeck for. They also changed the familiar yellow to blue.
I got rid of the new Tweetdeck and went back to the old one which still works like a charm. If you're looking for the old one, you're in the right place.
Just download the latest version and install. You will need Adobe Air first, so if you haven't got that, download it.
It will work with both Windows and Mac.