Chris

Hacking any corporate website (one that is seen by many people) and posting a message saying how you just hacked into their system, stating the how you did it and how to fix it. Also, that there is not way of them find you. This displays a sense of control over a known corporate entity, a vulnerability in their system and creates discomfort for that company. It is credible, since you are not lying or promoting something untrue about the company. And the people who are viewing the site, also get to see this. This would be great for a site that is based around their customers. For example a bank, because any customer that banks online and views this, see how the system is not 100% secure and this could definitely consider them to change their service. Or not banking online anymore. But a bank is so typical.