It is a tool for reengineering 3rd party, closed, binary Android apps. It can decode resources to nearly original form and rebuild them after making some modifications; it makes possible to debug smali code step by step. Also it makes working with app easier because of project-like files structure and automation of some repetitive tasks like building apk, etc.
It is NOT intended for piracy and other non-legal uses. It could be used for localizing, adding some features or support for custom platforms and other GOOD purposes. Just try to be fair with authors of an app, that you use and probably like.
JD-Eclipse is a plug-in for the Eclipse platform. It allows you to display all the Java sources during your debugging process, even if you do not have them all.
JD-Eclipse is free for non-commercial use. This means that JD-Eclipse shall not be included or embedded into commercial software products. Nevertheless, this project may be freely used for personal needs in a commercial or non-commercial environments.