Most of the major apps like Twitter and Facebook have already been redesigned to match the new look of iOS 7, and Apple has made it easier to hunt them down by creating a "Designed for iOS 7" section in the Apple App Store. The new section allows you to look through featured apps or search for a specific app by release date or name.
If you haven't upgraded to iOS 7 (or can't), you can still use the updated Twitter app, and get the latest Facebook update that adds the new bottom tool bar. Other notable apps that have also been upgraded for iOS 7 include Evernote, Zillow, NYTimes, Open Table and RunKeeper.