You can find RFC Reader here.
Description of the tool:
RFC Reader is a professional tool for reading RFC document. It can generate a section directory tree. And it provides lots of features to help you reading RFC documents, such as bookmark, highlight, search, and automatically record where you are reading. Version 3.2 is optimized and it is free now! You can use it without any limitation.
Another interesting tool is here.
It is minimal but quite good.