Apktool is a Windows/Mac/Linux utility for reverse engineering Android apps. It allows you to decode an app, change something, rebuild it, and pray it still runs. You're going to need something like this if you're into theming apps, hacking a feature onto someone else's app, finding security holes, or just want to hunt for info.
Apktool has been freshly updated to version 1.5.1, with the new headline feature being "Android 4.2 support." Here's the full changelog.