With older students in mind including those about to enter higher education, there are many free resources available to support students in their study of Mathematics.
Note that a new blog aimed at students is now available.
There are many sources of excellent notes online, a page on Mathematics for Students – Notes has an excellent collection.
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.
I would advise students of any subject to learn to use WolframAlpha. Suppose you wish to do some revision on techniques for differentiation for example – ask WolframAlpha to differentiate x2sinx, and the derivative will be returned with possible working available if ‘Show steps’ is selected. (Further Calculus examples). WolframAlpha provides an excellent way to check your work.
WolframAlpha will plot graphs, there are many other free and excellent online graphing tools, see the Desmos Graphing Calculator.
Thinking longer term – have a look at the excellent Maths Careers site.