Quanta is really a science magazine but has good articles of interest to math fans. Mathematical Digest is a magazine that has been produced for nearly 50 years now by Professor John Webb of the University of Cape Town. I grew up on these in high school and along with Martin Gardner's columns they fuelled my love of math. I was privileged to be instructed by Prof. Webb as an undergraduate math student as well; he was regularly awarded honors as a distinguished teacher and these were well-deserved.
Finally, there is Manifold, a student magazine of the University of Warwick, which is where the well-known math professor and writer Dr Ian Stewart hails from. I haven't looked at these yet but expect them to be interesting.
If you know of other sources of such magazines let me know in the comments!