Some starting points in the historyb of mathematics
Below you can find a collection of websites that I used to visit frequently: databases, digital libraries, and mathematical encyclopediae.
Databases of mathematical papers written after 1940
 Mathematical Reviews (MathSciNet)
[Database of mathematical papers with citations and abstract. Edited by the American Mathematical Society, founded by Otto Neugebauer in 1940. Universities have usually access to it.]
 Zentralblatt
[The German analogue of Mathematical Reviews, edited by Springer, founded by Otto Neugebauer in 1931. Universities have usually access to it.]

arXiv
[Preprint database of Cornell University. Authors usually publish their manuscripts here, before official publication in a journal.]
Collection of papers from the 18–20th centuries
 DML digital journals
[A list of digital mathematical journals with urls.]
 Göttinger Digitalisierungszentrum
[Digital library of the University of Göttingen. Lot of digital journals.]

Numdam
[French digital library of journals.]

Gallica
[Digital archive of the French National Library.]
 GallicaMath
Mathematical resources of Gallica.]
 Repository od the Hungarian Academy of Sciences
[The digitalization of Hungarian mathematical journals has begun.]
Digital libraries of books, journals and papers etc.
 Internet archive
[You can find almost anything form 1930. Books, films, music etc.]
 HathiTrust
[Digital library. Hathi, pronounced “hahtee”, is the Hindi word for elephant, an animal famed for its longterm memory.[]
 Open Library
[“One webpage for every book.”]
 Gallica
[Collected works of the greatest mathematicians such as Cauchy, Lagrange, Euler.]
 EuDML
[European Digital Mathematics Library: many digitalized journals.]
 University of Michigan Historical Mathematics Collection
[Collected works of mathematicians.]
 Cornell University Historical Math Monographs
[Digitalized mathematical monographs.]
 Google Books
[To look inside books legally.]
 LiNuM
[Collection of useful digital archives.]
 Collected works of
Leonhard Euler
[Original works with English translation.]
 Collected works of Bernhard Riemann
 Collected papers of Isaac Newton
[Handwritten works by Newton digitalized.]
History of mathematics
 MacTutor History of Mathematics
[Biographies of mathematicians. Maintained by the University of St. Andrews .]
 Earliest known uses of some words in mathematics
[With historical comments.]
 Earliest uses of various mathematical symbols
[With historical comments.]
Mathematical encyclopediae
 Wolfram Mathworld
[Full of interesting math: functions, online integrator, demonstration, browse!]
 Wolfram Alpha
[If you don't know something, just ask it...]
 PlanetMath
[Online mathematical encyclopedia.]
 Encyclopaedia of Mathematics
[Mathematical enycyclopedia of Springer.]