Obligatory:

• My math stackexchange profile
• School of Mathematics, University of Minnesota
• ISI Kolkata website
• ISI Bangalore website

Useful/interesting websites:

• Website to help drawing tikzcd diagrams

Mathematics blogs/websites:

• nLab
• Stacks project
• The n-Category Cafe
• Rigorous trivialities
• Terrence Tao
• John Baez