Below I list some websites that I visit very often. They are not very neatly categorized. I may do so at some point in time.
ACM Transactions on Algorithms
Advances in Applied Mathematics
Advances in Mathematics
Algebraic Combinatorics
Annals of Combinatorics
Bulletin of the Australian Mathematical Society
Combinatorial Theory
Discrete Applied Mathematics
Discrete Mathematics
Economic & Political Weekly
European Journal of Combinatorics
Forum of Mathematics, Pi
Forum of Mathematics, Sigma
Indian Journal of Pure and Applied Mathematics
Integers
International Journal of Number Theory
Journal of Algebraic Combinatorics
Journal of Combinatorial Theory, Series A
Journal of Integer Sequences
Journal of Symbolic Computation
Journal of Mathematical Analysis and Applications
Revista de la Real Academia de Ciencias Exactas, FĂsicas y Naturales, Series A Matemáticas
The Electronic Journal of Combinatorics
The Mathematical Intelligencer
The Ramanujan Journal
Theoretical Computer Science
Ahmedabad University
IIIT Manipur
Cardiff University
University of Vienna
The Abdus Salam International Centre for Theoretical Physics
Tezpur University
Gonit Sora
Arxiv.CO
Arxiv.NT
Arxiv.GM
Ron Graham Archives
Krishnaswami Alladi
George E. Andrews
Arvind Ayyer
Rupam Barman
Nayandeep Deka Baruah
Roger Behrend
Bruce C. Berndt
Gaurav Bhatnagar
Shane Chern
Mihai Ciucu
Hirakjyoti Das
Bishal Deb
Ilse Fischer
Ankush Goswami
Christian Krattenthaler
Tri Lai
Jeremy Lovejoy
Bibekananda Maji
Pankaj Jyoti Mahanta
Eshita Mazumdar
Anupam Saikia
Abhishek Sarma
Michael Schlosser
Florian Schreier-Aigner
James A. Sellers
Fernando Rodriguez Villegas
Terence Tao
Tim Gowers
Tanya Khovanova
Math with Bad Drawings
The value of the variable: H. A. Helfgott
Luit-Pariya
Peter Cameron
Igor Pak
British Combinatorial Committee
Academia Obscura
Viviane Pons
Mukunth V
How to do Math and TCS
Tips for Giving Clear Talks
How to referee a math paper?
Writing mathematics effectively