iOS: If you've heard about Siri, Apple's new voice-activated personal assistant for iPhone, and you wish more apps would speak aloud to you, TweetSpeaker is a new Twitter client for iPhone that will do just that. The app has easy-to-understand controls, and will read your stream, mentions, lists, or direct messages out loud to you.
TweetSpeaker supports AirPlay, so you can start it and let it read your stream to you through another set of speakers or your home entertainment system (not that you would want to-the voice synthesizer is apparently the same that the guys at Xtranormal use, so it's pretty funny to hear in that context,) and the app understands Twitter terms like "retweet" when it sees "RT." Here's a video demo of the app in action.
If you're addicted to Twitter, and wish there were a way to check it while running or on a long drive without actually touching your phone, TweetSpeaker can help. It's $2.99 in the iTunes App Store.