I don't know what you're asking for, so I'll just say G2play.net.
I just wanted to know which one is safer
how do they send you your keys?actually how do extra keys in steam work to begin with?
What do you mean? If the game operates using Steamworks, aka is based on Steam, then you can enter it's CD key code into Steam and it will be activated with it. Games like Modern Warfare 2 and 3, Dawn of War II, Space Marine, or Batman Arkham Asylum/City work like this.
lets say i buy 2 copies of saints row 3, from one of the cd key sites, could i gift the second copy to a friend?
I think so, probably. Not entirely sure about it but it's likely.