If you use Twitter's official Mac app on a MacBook Pro with retina display, you know that it sucks. It's a piece of unreadable blurry crap that makes me want to punch my computer. Thankfully, Tapbots came to the rescue with Tweetbot, a gorgeous, retina-optimised Twitter client.
Seriously, Mac developers, you need to get the retina versions of your apps up to speed, because some of them are really not looking good.
Tweetbot, on the other hand, is really pretty and shines on the retina display. It's an alpha version, so it may boom and bang here and there, but so far it's been solid for me. Get it at Tapbots.