The page level above is math resources.
Most of interesting people for nlab will have their own pages in lab; many of them keep their own articles and books there, but some do more. Thus this will not be a general list of homepages of mathematicians, but rather of those pages which have particularly important or large collections of articles, books and especially advanced lecture notes of major importance to lab and in the areas of prime importance to lab (sheaves, stacks, (higher) categories, modern geometry and homotopy theory, algebraic topology, foundations, mathematical physics). Still let us keep the alphabetic order (by family name)
Dror Bar Natan keeps an archive of his notebooks
groupoid homepage has various info on research on groupoids including list of addresses
Paul Smith’s noncommutative algebra and geometry page has a large list of links to homepages of geometers and algebraists
David Mumford, keeps lots of his online material
Pointers to several similar collections in answers to MathOverflow question free-high-quality-mathematical-writing-online
The Assayer, a catalogue of freely available mathematics books in electronic formats (also computer science and other fields). Some books are even reviewed.