Cyanotype is also a very old process, invented in 1842 by Sir John Hershel. It is an iron-based process resulting in images compose of the pigment Prussian Blue. The native blue color can be shifted to various shades or brown and purple by toning them in various reagents. This comes at the cost of some stability. Native cyanotypes are among the most stable images known; toning them seems to reduce that.
Since cyanotype does not involve any precious metals it is one of the least expensive of the alternative processes. Thus, this process is usually how folks are introduced to this photographic niche.