EuDML (The European Digital Mathematics Library)
Google Scholar (Stand on the shoulders of giants)
inSPIRE (High-Energy Physics Literature Database)
Bateman Manuscript Project (Formulas for special functions)
nLab (A wiki-lab for collaborative work on Mathematics, Physics and Philosophy)
Encyclopedia of Mathematics (An open access resource designed specifically for the mathematics community)
The Electronic Library of Mathematics (ELibM) (The longest-running and largest open access repository in mathematics)
The Wolfram MathWorld (the web's extensive mathematics resource)
Abramowitz and Stegun. (Handbook of Mathematical Functions.)
The Heun project (The project for the Heun functions, their generalizations and applications)
LMFDB (L-functions and Modular Forms Database)
Math On The Web (Mathematical Literature on the Web)
The Stacks Project (An open source textbook and reference work on algebraic stacks and the algebraic geometry needed to define them)
References on multiple zeta values and Euler sums (A list of the references on the subject)
The online database of Vertex Operator Algebras and Modular Categories (An online database of VOAs)
Algebraic topology: A guide to literature (Japanese website for some database in algebraic topology)
Cinderella (Intractive geometry software)
CinderellaJapan (Cinderella in Japanese)
Euler Math Toolbox (A powerful math program combining numerical and symbolic tools)
LiE (A computer algebra package for Lie group computations)
LieART (a Mathematica application for computations frequently encountered in Lie algebras and representation theory)
Magma (A computational algebra system)
Maxima (A computer algebra system)
Nemo (A computer algebra package for the Julia programming language)
R (R is a free software environment for statistical computing and graphics)
SageMath (A free open-source mathematics software system licensed under the GPL)
SciLab (Open source software for numerical computation)
Singular (A computer algebra system for polynomial computations)
Yacas (Yet another computer algebra system)
Oberwolfach References on Mathematical Software (A reference for mathematical software)
NZMATH (Python calculator on number theory)
MathOverflow (A place for mathematicians to ask and answer questions)
Mathematics Stack Exchange (A place for people studying math at any level and professionals in related fields)
mathtube.org (The lecture videos and lecture notes organized by PIMS)
The Mathematics Genealogy Project (Find your mathematical ancestors!)
MathTree (Check your Ph.D. Tree!)
The University of Michigan Historical Mathematics Collection (A digital collection of math papers)
The Cornell University Library Historical Mathematics Monographs (A digital collection of historical math monographs)
The On-Line Encyclopedia of Integer Sequences (Find interesting sequences!)
The History of Mathematics archive (Let's learn about the history of mathematics!)
OpenStax (Free textbooks)
Math3ma (Interesting math blog)
Alan Schoen geometry (Interesting pictures!)
Graphics Library of Special functions (Graphics of special functions!) (Japanese only)
A Video for Möbius Transformations (You Tube) (It's a cool video.)
Beall's Llist of Predatory Journals and Publishers (Don't publish in fake journals!)