Microsoft Visual Studio Community 2013 can be used to Developed NAV 2013 R2 Reports. Visual Studio Community 2013 includes all the great functionalities of Visual Studio Professional 2013, designed and optimized for individual developers, students, open source contributors, and small teams. Please download for free from here
I was able to open report layouts of NAV 2013 R2 with this edition and made some modifications. It looks like this is the Free IDE for NAV 2013 R2 Report Development – an alternative to Visual studio 2013 professional which is not free. Check it out.