There are a number of different specifications depending on whether you are sitting the exam before or after 2011, and whether you are doing the linear or the modular course.
The starting point to look up the paper you want is the AQA GCSE Mathematics page. When you get there click on the course you are interested in.
For the old courses, you then the "Key Materials" tab / "Past papers and mark schemes" / "Select a series" / Then click to download the past paper or mark scheme you are interested in.
For the new courses, after clicking on the title of the course, select the "Assessment" tab then click on the specimen paper or mark scheme you would like to see.
Edexcel do not make their most recent past papers available, but papers up to March 2010 can be located as follows:
To locate past papers for November 2007 or earlier, go to the Past papers for free download page. Click on the exam date you are interested in (for some dates in the list there is no maths exam), then on Mathematics, then choose the Question Paper or Mark Scheme you want to download.
To locate the most recent papers avaiable, go to Question Papers and Mark Schemes page. Click on "GCSE from 2001" from the "Select a qualification family" box (there are no maths papers available in the "GCSE from 2009" bit), then click on "Mathematics Linear" or "Mathematics Modular", and then select your document type ("Mark Scheme" or "Question Paper"), then the "examination series" -- ie the date, then the actual paper.
Past papers are available for the old specification which expires in January 2012. To get a list of past papers and mark schemes available for download, click the link for your course.
WJEC past papers are available on their Mathematics GCSE WJEC page.
Mark schemes are available for purchase from their Online shop