Here is list of some important mathematics blogs, magazines and other websites.

Blog:

What’s new (Prof. Terence Tao, Fields Medalist)

Gower's Weblog (Prof. Timothy W. Gowers, Fields Medalist)

Alex Bellos's Adventures in Numberland (Alex Bellos)

Division by Zero (Dave Richeson)

Geometry and the imagination (Danny Calegari, California Institute of Technology)

Tanya Khovanova’s Math Blog

MAA NumberADay

 

Magazine:

Gonit Sora (English, Assamese)

Plus Magazine

The American Mathematical Monthly

Convergence

The Intrepid Mathematician

 

History:

MacTutor History of Mathematics archive

Jeff Miller Web Pages

 

Note:

Math Explorers' Club

 

Guide To Understanding:

Khan Academy

"Complete Guide To Understanding The Importance Of English & MATHS In Education & Beyond"

Skillswise

Skills Workshop

 

Forum:

Art of Problem Solving

The Math Forum

 

Encyclopedia:

Wolfram MathWorld

Numericana

Interactive Mathematics Miscellany and Puzzles (Cut the knot)

MathPages

 

Resource:

The Prime Pages

Number Theory Web

 

Foundation:

Clay Mathematics Institute

 

Organization:

International Mathematical Union (IMU)

 

Society:

American Mathematical Society

Mathematical Association of America

 

Others:

The Abel Prize

International Mathematical Olympiad

 

List of Mathematics-Related Sites:

The Net Advance of Physics

Theorem of the Day

 

Mathematical Societies in India

Mathematical Societies in Asia Pacific Region

Institutes Offering Mathematics Courses India