Alex Novosad has released Choosy, a framework for developers that allows iOS users to decide which app should launch when a link is tapped, though only in those apps that have incorporated the framework. In other words, if you're using a Twitter client that has used this framework, a user could decide to automatically open URLs in Chrome or Dolphin instead of Safari.
Here's a video from Mr. Novosad explaining Choosy:
As suggested by iMore, this has the potential to be a big change in iOS. Suppose you prefer a different mail client than the one that ships with iOS. This way when you tap the mailto link, you get a choice which app will launch.
This would work in a similar fashion to how Launch Center Pro and Drafts handle things, using URL schemes that offer an option for which app should be used. Any app that wants to include the framework can do so, according to the announcement.
Currently this is a beta framework, available on GitHub for anyone to deploy as part of their app. It isn’t implemented by Apple, so it doesn’t work everywhere, and next month’s presumed iOS update could have its own new way to determine default apps, but in the meantime this looks VERY cool.