SecureWorks' Security Research Group has developed several security tools that are publicly available. All tools are released under the GNU General Public License (GPL) and are provided "as-is", with no warranty and no support.
Before going into the GWT specifics, the article gives some useful background on AJAX security issues including Same-Origin Policy, Cross-Site Scripting, Cross-Site Request Forging, and JSON.
Bruce Schneier links to a paper on a JavaScript vulnerability that affects multiple browsers. It is easiest to exploit when JSON is used, but commentors maintain other attack vectors can also be used.