If you are developing ubercart / drupal then you need to test your creditcard checkout system. There is no better way then to do it with those 2 numbers :

Visa:        4544182174537267
Master Card: 5460506048039935

ATTENTION : These credit card numbers are not real! Looking for the security code? Just type in 3 random numbers and make sure the expiration date is greater then the current day

Thanks to The blog of benjisimon