Notice how most "free trials" require you to provide your credit card number? That's so the company can charge you a monthly fee once the trial is over. Many of these services don't let you cancel online, either -- you have to call the company on the phone and argue for 20 minutes with a person whose job security depends on their ability to prevent you from canceling.

Here's an article that shows you "How to Get Unlimited Free Trials Using a 'Real' Fake Credit Card Number." (I don't know if this is legal or not, but I like it)