Apollonius of Perga (c. 262-190 B.C.): known as 'The Great Geometer', great influence on the development of mathematics, famous book Conics introduced terms such as parabola, ellipse and hyperbola.
Desargues - Girard Desargues (1591-1661): Founder of projective geometry, his work centred on the theory of conic sections and perspective.
Heron of Alexandria (ca. 75 AD): Greek mathematician who was mainly interested in practical studies in mechanics and engineering, best known for Heron's Formula
Klein, Felix (1849-1925): Plücker's assistant at Bonn who studied Analytic Geometry, describing geometry as the study of properties of figures which remain invariant under a Group of Transformations. He systemized Non-Euclidean Geometry.
Monge, Gaspard - Conte de Péluse (1746-1818): The father of differential geometry, he devised a system called Geometrie descriptive, now known as orthographic projection, the graphical method used in modern mechanical drawing.
Riemann, Georg Friedrich Bernhard (1826-1866): Theory of functions, non-Euclidian geometry, relativity theory, Famous for the Riemann hypothesis about the behavior of zeros of the Riemann zeta function, which he showed determines the distribution of prime numbers.
Steiner, Jakob (1796-1863): Was one of the greatest contributors to projective geometry.