Unfortunately Library Genesis is down and has been down for some time now, so I'm taking the liberty of editing this answer. A working site that is similarly useful is libgen.info and someone has collated a blog of links here.
Original answer follows:
At the Library Gensis or the translated version here, you can browse and download as many high-quality and modern Mathematics books, surveys, etc as you wish. This i-resource must be on every mathematician's i-shelf.
Here is some list:
599 books on Number Theory;
303 books on Complex Analysis;
325 books on Algebraic Geometry;
588 books on Partial Differential Equations;
97 books on Abstract Algebra;
107 books on Commutative Algebra;
181 books on Harmonic Analysis;
133 books on Fourier Analysis;
349 books on Functional Analysis;
356 books on Differential Geometry;
88 books on Riemannian Geometry;
783 books on Topology;
286 books on Combinatorics;
323 books on Graph Theory.
This is enough for illustration. You will find more, enough to get you in a downloading craze!
