RepRap is the first open source 3D printer. In February of 2004, the RepRap blog was born, beginning one of the most powerful and influential movements in modern manufacturing.
Short for "replicating rapid prototyper", the RepRap 3D printer has literally evolved since its official release in 2005 to 8 official versions, each with many variations. It is both an open source 3D printer and a project. Some refer to the ideals of the project as "The RepRap Movement". Its design is completely free and open source (download here) and encourages users to create their own modifications as well as print out parts for those who wish to build their own.
Both the printer and project were created by Adrian Bowyer in 2005. The University of Bath in England helped to fund Bowyer's research in 3D printing. At the time, he was the Senior Lecturer of Mechanical Engineering at the school and was involving students in developing the RepRap.
According to a study in 2012 by SSPP, RepRap is still the most popular open source printer in the "Maker" community. The RepRap movement is interested in a radical shift in the consumer model we currently live in: One that puts more power into the hands of the product user.