Although very simple, I have published a new small portable class library that adds some extension methods to the BinaryReader and BinaryWriter classes. It consists of overloads to the Read* and Write methods to allow switching the endianness of the data being readen/written. Since almost all current .NET CLR platforms run on little endian (Windows Phone does too, thanks to the ability of ARM to switch between big and little endian), these functions are helpful when reading data exported from other platforms, or more commonly, JPEG files.
The library is available for download from GitHub, both in source code and compiled form. Or if you prefer, you can search for the BinaryEndiannessExtensions package on NuGet and add it directly to your project.