If you see a message telling you about a code-generating website, DO NOT open it!
Some users on the PSN have had their accounts hacked and charged with several hundred dollars' worth of PSN content. What do they have in common? They have opened PlayStation Network messages with URLs leading to "code generator" websites, which claim to provide free PSN content.
Needless to say, this is a scam, and a very dangerous one. Open one of these messages and you could find your account hacked and charged a huge amount of money.
How the spammers/hackers have gained access to the accounts of the victims is uncertain.
If you believe you have opened one of these messages, act quickly.
1) Go to
http://playstation.com
2) Sign in to your account
3) Click on "Edit My Account Settings"
4) Log in again
5) Remove your credit card information
6) Change your email and password
Check your email to see if you have been charged for anything you did not purchase. If this is the case, call Sony and your credit/debit card company and explain the situation.
If you have not yet opened one of these messages and receive one, do not open it. Simply press Triangle while highlighting the message, and delete it. Make a habit of deleting chain messages and messages from people whom you do not know.
Thanks to Iron Star Movement for the tip.