Birdhouse is a notepad for Twitter, a place to collect and edit your tweets before you actually put them online. It's designed to sit alongside your favorite iPhone Twitter client as a repository for your more polished one-liners. It's wholly unnecessary for anyone who doesn't care to ever think before they tweet. $4.
More than anything, though, at least for today, it's the app with the most amusing introductory video of all time. For today. (I should have edited that.)