Here you can create custom XSS callback URL's for penetration testing. You may create up to 3 XSS callbacks per Google Account.
How does this thing work?
Simple. You create a custom callback URL on this site and customize it. You can make the callback an image, a re-direct or a plaintext file (Limited in size). You then use your sites XSS exploit to callback to your custom callback on this site - which then logs the data from the callback. What data you log is also customizeable.