Additive fabrication technologies have been based on just about every form of matter known to man. Even gases have been used as a starting point to form small objects. Some idea of the wide variety of approaches that have been utilized can be found on Henri Koukka's Whole RP Family Tree web page. Object sizes currently addressed by these technologies range from microscopic to entire buildings. Materials range from paper and plastics, to metals and ceramics.
Applications range from toys to aerospace and advanced medicine.
There are about thirty companies worldwide that produce rapid prototyping systems. Many of these companies have close relationships with RP material suppliers, and about twelve of them make systems that are based on some variation of the stereolithography process. As the systems evolve further and the materials they use become able to accommodate a wider range of desirable mechanical properties, the term, rapid prototyping will become even more of a misnomer. This technology really represents the nascent state of computationally-based manufacturing. The real world is tangible and rapid prototyping is the link between Moore's Law and the tangible world.