Acceleo is an open source code generator developed inside of the Eclipse Foundation. As such, you can use it freely, fork it and even contribute back.
Acceleo is deeply integrated in the Eclipse IDE with a full fledged editor with syntax highlighting, real time error detection, quick fixes, refactoring and much more. It also comes with dedicated views to help you navigate within your code generator and quickly access code generation design patterns.
Whether you are considering it or not, you will one day have to modify manually your generated code and you want to keep your modification even if you are regenerating your code. Acceleo lets you define protected areas in which you can safely modify the generated.
Acceleo does not lock you inside of the Eclipse environment, as such you can build and run your generator easily out of Eclipse. Acceleo provides a brand new Maven integration.
Code generators are often limited to a set of technologies. With its template based approach, Acceleo can generate code for any kind of languages. If you can write it, Acceleo can generate it.