Here are some links I find interesting or helpful, or both. Listed in no particular order.

• HaskellForMaths
• Open Problem Garden
• AMS open notes
• Macdonald polynomials webpage
• Hacker News
• Armin Straub
• Bill Cherowitzo
• Richard Stallman
• Aaron Swartz - The Internet’s own boy
• False, Misleading, Clickbait-y, and/or Satirical “News” Sources
• Differential Equations & Linear Algebra - Lecture notes on the web
• William Stein - William Stein, the creator of SageMath
  • The origins of SageMath Stein’s BP centenary talk at Harvard
• SageMath - Open-source maths software system
  
• Project Euler
• Blockly games
• Math ∩ Programming
• Authorea
• Big Data
• Fermat’s Library
• Tricki
• AMS Feature Column
• arXiv
• What’s new - Terence Tao’s blog
• Gowers’s weblog - Timothy Gowers’s blog
• Polymath - MMO maths research
• OEIS - The On-Line Encyclopedia of Integer Sequences® (OEIS®)
• Vi IMproved - the one true text editor. Plugins:
  • vim-latex - for latexing
  • vimwiki - a wiki tool with google wiki-like markup
  • neocomplete - for auto-completion
• vimperator - turn your Firefox into Vim
• muttator - turn your Thunderbird into Vim
• zathura - turn your pdf reader into Vim
• i3wm - turn your window manager into Vim
• VimGolf
• Regex Golf
• Regex Crossword
• Arch Linux
• Jupyter notebook - An open-source notebook
• Stackexchange sites
  • Mathoverflow
  • Mathematics
  • Codegolf - The most fun corner of Stackexchange.
• Danial Bump
• Bill Casselman