Unfortunately, there is no such free tool to repair/recover NSF file. Although the demo version of such tools are available for free but they only show the recoverable components of a corrupt NSF file. To save the recovered NSF file you need to buy the license fo the tool. One such NSF file recovery tool that shows 100% preview of the recoverable components is Stellar Phoenix Lotus Notes Recovery. This tool is very easy to use and is able to repair NSF file from any instance of corruption.