Riemann's work with curved spaces, particularly their applications to physics was at least 40 years ahead of it's time. He pushed forward ideas that space is perhaps curved and that forces such as gravity could be thought of as bending in space. He gave a few lectures on these ideas, but fellow mathematicians and physicists didn't really know what good could come of them and didn't pay much attention. Of course, Einstein finally solved the puzzle many years later.
Also, Joseph Fourier was laughed at when he proposed the notion of Fourier series for solving the heat equation, particularly at the lack of rigor and the overall scope of it's applications. Opinions on the matter changed a decade or two later when the theory began to root itself in rigor thanks to Dirichlet.
