The page level above is math resources.
Most of interesting people for nlab will have their own pages in $n$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 $n$lab and in the areas of prime importance to $n$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)
David Ben-Zvi has a nice geometric Langlands page and of many other lecture notes
Dror Bar Natan keeps an archive of his notebooks
groupoid homepage has various info on research on groupoids including list of addresses
Jacob Lurie’s website has the latest editions of his papers (most of which are on arXiv but not all) of large importance to the subject of $n$lab
James Milne’s math page has several books and lecture notes, mainly on algebra and algebraic geometry
Daniel Murfet’s The Rising Sea is classified under math blogs but has large amount of neat and modern lecture notes on algebraic geometry and homological algebra
Andrew Ranicki’s homepage has several online books, slides of a number of talks and links to some papers and some other material like the Maslov index seminar page with nice bibliography
Bodo Pareigis has here several lecture notes on category theory, quantum groups, noncommutative geometry and Hopf algebras
Paul Smith’s noncommutative algebra and geometry page has a large list of links to homepages of geometers and algebraists
Charles Weibel has here lots of writings including a growing book on algebraic K-theory and lecture notes on motivic cohomology with Mazza and Voevodsky
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.
hundreds of mathematics questions and answers free-high-quality-math-practice-online