Added today is the Polarizers and the M System article.
The article discusses several solutions to allow you to use a polarizing filter with an M camera - something that's not necessarily as easy as you might think.
Added today is the UV/IR Filters article.
The article discusses the problem necessitating the use of the filters, the various manufacturers that make them, and their use on various cameras.
Added today is the Viewfinder Magnifiers article.
The article discusses why you would use one, and covers the various manufacturers that make them, with the pros and cons of each.