Plus Magazine (University of Cambridge)
The Maths Careers site has sections for ages 11 through to adult.
Platonic Realms includes a daily ‘Math Moment’ and a daily challenge.
Students going on to study Mathematics degrees have a look at Kevin Houston’s ‘How not to get a good mathematics degree‘ and ‘How to get a good mathematics degree‘. He also has provided a pdf file you can download: 10 Ways to Think Like a Mathematician. Kevin Houston works at the University of Leeds.
See the Reference page also which includes a useful list of reference materials.