1. Free Exam Papers
This has pretty much everything. It might not have the very latest papers (i.e. the last year or two) but the range of exam boards that it covers is huge. GCSE, iGCSE, IB, A Level, SAT.. You name it, they have most of the past papers here!2. Free Past Papers
This site is still in the making, but it is making promising progress. At the moment, it has full SATs (KS1, KS2 and KS3) and the GCSE papers are in beta. They currently only have the GCSE Maths papers at the moment, but expect the other papers to come shortly.
The site was only launched in January this year, but they have come a long way since then and are attracting a healthy number of visitors! They are anticipating everything to be ready by 2013, so keep an eye out for them.
3. Carl Gauss
This is my personal favourite for Maths past papers for A Level and iGCSE. It has a neat interface, and has most of the content too. It's missing the higher end papers such as FP2 and FP3, but I really like the layout of the page.
XtremePapers is my favourite site for past papers. The site is appealing to the eye in its archive-like manner and it really has everything I ever need. This site has hardly ever let me down. I highly recommend this site.5. Maths Papers
The heading explains this site pretty well. It is a site of only maths papers for A Level, for Edexcel and AQA exam boards. It has all the modules you will need though, and it is the one I used most for A Level Maths. The only downside is the slightly cheap-looking interface, but that shouldn't really matter, because the content is brilliant.,