It depends on the subject.
For english it's nothing extra
For modern i just brought the HTA book.
For 3U maths i have the Jones & Couchman text book and my tutor ^^
For 4U maths it's excel, extra stuff my teacher gives me and my tutor
For extension history it's completely what my teacher gives me (there's heaps)
For Physics it's Creelman HSC Exam Questions and excel.
For Chem it's Excel
And for all of the above i have Success One for every subject possible and the excel pocket forumla book for ever subject possible.
I reckon that's more than enough. Most of them are 2nd hand or are pretty cheap so i didn't spend that much on them. They do help though because i like to do questions from random books ^^
Don't spend too much though, i know heaps of people who had a whole collection and never use most of them.