A Treatise on Electricity and Magnetism by James Clerk Maxwell is one for physics.
Although I've never read his work, http://en.wikipedia.org/wiki/Hermann_Grassmann" wrote two major papers/texts, (1) The Theory of Linear Extension, a New Branch of Mathematics and (2) The Theory of Extension, Thoroughly and Rigorously Treated. In these he created linear algebra (and in the form we know it today), and his work lead to the later development of differential forms and exterior algebra. This work is significant because although revolutionary, it was ignored his entire life, never credited, and he remained a high school teacher, never attaining a professorship.
Also, Analysis Situs by Henri Poincare, a treatise on algebraic topology.
There's probably something by Alexander Grothendieck that deserves to be on this list, but I'm not familiar enough with his work. His work is also rather fragmented due to his isolation.
Although it there is a natural feeling that only works that created whole fields (which is rare) should go on here, Grigori Perelman's proof of the Poincare Conjecture is a huge feat of mathematics.
And last, but not least, Calculus by James Stewart...hahahahaha.
