Fake Credit Card Numbers

Today I registered for a local engineering conference. Although my registration was free, for some reason their eCommerce site wanted a credit card number. Well, I don't particularly care to give my credit card out to just anyone on the internet, so what do I do, and why does their site want my credit card number if they aren't planning to charge it for anything?

Of course, I did the only reasonable thing a security concious engineer could do, I did a quick search for "fake visa number generators" on the internet and found this gem of a site:

http://www.darkcoding.net/index.php/credit-card-numbers/

Then, I took the first number off of the list, made up an expiration date and CVV number, and tried it out. Two pages later I got a lovely confirmation stating that my registration had been accepted.