French physicist, 20th century. Proposed that all matter can behave like a wave. The object's wavelength, lambda, is calculated by

lambda = h/p

where h is Planck's constant and p is the particle's momentum (mass*velocity). Note that for very large objects (anything you can actually see) the wavelength is insignificantly small (10^(-34) metres, give or take several powers of 10) but for something as small as an electron you get something around the 10^(-12) metres range, which corresponds to the spacing between atoms. This leads to some very interesting effects such as electron diffraction, a technique commonly used to study crystal lattices.

De Broglie's hypothesis eventually became the basis of wave mechanics and an important step forward for early 20th century physics.