Humble Bundle keys?

Because of this, and other similar statements: http://support.humblebundle.com/customer/portal/articles/243221-can-i-sell-give-away-my-keys-

Of course, the gift links make that seem weird, and if you contact support about the issue they seem to indicate that as long as you are not reselling the keys and are privately giving away unwanted or duplicate keys they're not actually terribly bothered by it. So, yeah, a definite grey area as it turns out, since they technically don't allow it but also make it possible and don't terribly mind it.

You're allowed to give them friends. That's not the same as giving them to random people on the Internet, and especially not if giving them away gets you some sort of benefit.