Mac: It has been a long time since Twitter bothered to do anything with their Mac app, let alone anything substantial. This time around, the app's actually got some legs on it, supporting a variety of newish Twitter features. The new app supports many of the various Twitter features that have rolled out to the web app and mobile versions over the last year. It now supports inline video playback, GIFs, group messages, tweet quoting, and the mute feature. Alongside support for a now archaic list of features, it comes with a new design that fits in with Yosemite/El Capitan, a dark theme, and a Today widget. The app still lacks basic features like support for Twitter polls or split screen support in El Capitan, but at least it's not obvious abandonware anymore.
Twitter (Free) [Mac App Store]