You just never know when you'll need a fake credit card number - and when you do, here's a site can provide them for you.
This probably sounds like a sketchy idea, but it's actually quite legit - you often need a "valid" credit card number to test out payment functionality. Which is why I needed them this morning.
As a short cut, here's a few numbers to get you started:
Visa: 4544182174537267 Master Card: 5460506048039935
Remember, you can't actually use these, and they aren't stolen or linked to real people - they just pass the test that Credit Card numbers need to pass to look Kosher.