iOS: If you're a Twitter fan looking for a way to keep up with your tweets while driving or exercising, Tweet Speaker will read your tweets out loud. You can hear your regular timeline, mentions, or any of your lists; hit play and you'll hear your tweets.
The interface is great and the app works more intuitively than you'd probably expect, but at $2.99 it should be polished. You can hit play to begin reciting your tweets, or turn the dial back to a specific time; this seems like a great way to catch up on missed tweets when you get back into town from a goliday. Other notable features include the ability to adjust the reading speed and syncing with other Twitter apps so you can automatically catch up where you left off.
With the abundance of useful free apps it takes either unique functionality or an extremely polished interface to shell out money, but I think Tweet Speaker delivers on both of these criteria (and at this writing it is also on sale for $0.99). Give it a shot if missing tweets gives you the willies.