When I see the words "free trial," I know I'm probably going to have to whip out my credit card and enter in the number to "not get charged." Then I end up forgetting about the trial and want to kick myself in the ass when I see my statement at the end of the month. For that to work, you would need a valid expiration date and CVV or CSV number.
In order to avoid that rigmarole, you can actually use fake credit numbers instead of your own, and you can do that using the site getcreditcardnumbers.com, which can generate up to 9,999 credit card numbers at a time, or just one. This site merely provides the standard 16 digit credit card number that can be used to bypass certain online forms that only ask for the number.
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.
You can, however, get unlimited free trials on those sites using a simple trick with your email address if you have a valid card number with expiration date and CSV.
There's also an Android application for getting fake card numbers called Card Gen, available for free in the Play Store.
You can generate and validate credit card numbers directly from the app, making it easy to use on the go as well.
Validation in particular would be useful if you were accepting credit card payments on your own site and wanted to make sure the cards were legit.
The app is ad-supported, but since it's free, I can live with that.
In the generate field you can select from most of the major credit card providers, including American Express, Mastercard, Visa, and Discover.
The disclaimer explains what the app does and how you should use it.