Well there's this site that I think I actually got of Bos somewhere (thanks to the person who posted it, forgot who now). Its got lots of great links to notes etc. for learning maths. Most of it is University maths, and so is a bit beyond the average HSC student. There are however some interesting things that HSC students can do in there. The website is:
http://www.geocities.com/alex_stef/mylist.html
You're in Year 11 by the looks of it right? Well if you want to do four unit maths, then learning a few things in the syllabus, or at least looking at them before you see them in class can be really useful. A quick look at polar coordinates, the basics of Complex Numbers, and a bit of experimentation with graphs can go a long way to help in the transition to four unit, and it is also pretty interesting stuff. If you're not doing four unit next year, then the same principle applies for all the maths subjects. If you take a look at next years course, and try some of the basic stuff, then it gets so much easier in class when you actually go to learn it. If you manage to learn some of the concepts, then you can extend that when you do learn them in class.
All in all, a list of topics that i think are interesting/useful to learn would be (and by no means a thorough list, just a few i know i found interesting):
Complex Numbers (the basics, (-1)^1/2 = i etc)
Polar Coordinates
Vector calculations (if you haven't done it already in physics/ext1 maths)
Probability (only because i suck at it, and i wish i had done a bit more study on it before now. Its not interesting, but it is useful...sometimes.)
Thats all from me for now. Hope it helped.
