I've found this article on dpreview about landscapes:
and they talk about filter systems from lee or Cokin.
two major square filter systems used nowadays are: 85 and 100. numbers refer to the size (width in mm) of filters.
lot of photographers refer to 85 as simply "Cokin P" or just "Cokin", although the company originally started with smaller filters (A-size, 67mm) and currently manufactures 4 different formats, including 100mm. Cokin is an inexpensive option to build a fine set of hardware and explore possibilities of square filters.
Lee on the other hand was developed for professionals and for use with big cameras/wide lenses. filters start with 100mm and are much more expensive than Cokin. I've personally never used this system so can't comment on quality or durability.
middleground for me is the english brand Hitech - they manufacture filters for 85 and 100 size (nowadays even more), with various tastes and possibilities. their manufacturing quality is very much OK, significanlty better than Cokin. price is also higher than Cokin, but at least in Europe its well below Lee.