Link for the Online Encyclopedia of Integer Sequences. Chances are if you encounter sequences of natural numbers in your work, this website will become your best friend.

Link for Wikipedia. The amount of informations you can get about mathematics is enormous, and it is also good to find references and keywords.

Link for the mathematics section of StackExchange. This is a Q&A website for general mathematical questions, where thousands of people are susceptible to know the answer to your questions (and are willing to answer them!).

Link  for Mathoverflow. This is the research level version of Math.StackExchange. This website provides a place where mathematic experts can discuss about specific questions.

Link for The Stacks Project. This is a website with a ton of open source material about algebraic geometry, but you can find way more than algebraic geometry-related mathematics. In particular, chances are that the material of any course at the undergraduate or graduate level with an algebraic flavour you follow is covered in this website. It is also a very good reference for basic topology.

Link for the nLab. Another great website for algebraic flavored topics. It is very practical to find references about specific advanced topics in algebra, in particular anything related to category theory.

Link for Quiver. A website which allows one to draw commutative diagrams with ease and a good range of options, and to automatically translate these diagrams into Tikz.