I spent a few years running an online and mail order shop, and wrote this utility for my own benefit. One of the big problems retailers face when handling mail orders is credit/debit card numbers that are badly written by the customer, so you're not sure if what you've got is a valid card number before trying to process the order.
This is a simple utility which will check a credit card number and tell you whether it's a valid card number or not and, if so, which type of card it is. All it does is calculate the check digit and perform some format checks against the number itself - it doesn't make any connection to the merchant system - so it won't tell you whether there's any money in the account, or check the customer name/address, or anything like that. But it's still a handy basic sanity check if you think you may have a card number that's been incorrectly recorded.
Important Disclaimer
This facility is accurate for the majority of cards issued in North America and Europe, but it is not 100% complete and is more likely to be inaccurate for cards issued in other parts of the world. Unfortunately, there is no publicly available, up to date list of all card issuers and check data - the only organisations which have a full list are the card alliances themselves (Visa, Mastercard, etc), and they don't publish it. Even the card processing companies and banks don't have a full list, since all that matters to them is the abiliy to refer the card to the correct backend processor. New card issuers and card number ranges are assigned on a regular basis, so the database that this facility is based on is always going to be at least slightly out of date as well as incomplete.
You should not, therefore, rely on this facility for any commercial purpose - or, indeed, any purpose at all other than mere curiousity. By using this system, you accept full responsibility for any decisions you make as a result of the data provided.