The following are a few of the numerous online magazines that are devoted to mathematics:

Convergence

http://mathdl.maa.org/mathDL/46/

Sponsored by the Mathematical Association of America (MAA) in cooperation with the National Council of Teachers of Mathematics. Emphasis is on teaching mathematics using its history.

The Euler Archives

http://eulerarchive.maa.org/

A publication of the Mathematical Association of America (MAA), it is a digital library dedicated to the life and mathematical studies of Leonhard Euler.

Infinity

http://www.maths.uq.edu.au/~infinity/

Online mathematics magazine from the University of Queensland, Australia.

NRICH

http://nrich.maths.org/about

Put out by a group of qualified mathematics teachers for math students 5 to 19 years old; it is also part of the Millennium Mathematics Project in the U.K.

Pi in the Sky

http://www.pims.math.ca/pi/

A periodical designed for high school students in Canada. Published by the Pacific Institute for the Mathematical Sciences.

Plus

http://plus.maths.org/index.html

This online magazine is a part of the Millennium Mathematics Project, a United Kingdom initiative based in Cambridge. Its aim is to introduce readers to the practical applications and beauty of mathematics.