of the "many eyes" which belong to hackers and whose attacks might exploit (and make public) security flaws.
Working with phpWS a while back, a series of sites got paved. The hacker was polite enough to explain where the security hole was, how it was exploited, and give suggestions for how to fix it. He had his fun, we got a stronger system.
I don't know of many proprietary systems which have this kind of resource. But then, we didn't threaten him with jail time and punitive civil judgements.