Now, to be completely clear, these numbers cannot be used to purchase any item. The credit card number generator uses a system based off of the Luhn Algorithm, which has been used to validate numbers for decades.You can learn more about the algorithm on their webpage, or check out the Null Byte guide on how credit card algorithms work.
There's also an Android application for getting fake card numbers called Card Gen, available for free in the Play Store.
Or do you want to play the rewards category game to earn more rewards?
If you choose the latter, try to budget out how much, on average, you spend on certain rewards categories.
A fake number will work for sites that store credit card information to either charge you later or ask you to upgrade.
For sites that ask for an upfront fee or have an automatic charge sometime down the line (Hulu Plus, Netflix, Spotify), this won't work since they ask for more than just a credit card number for validation.