So, of all the races, humans are the least deserving of an inherent skill. However, I think I've come up with one that doesn't seem like it would be too overpowering.

Charm. Humans could invoke it an gain a small bonus in Charisma for a while. Similar to the effects of beautify without being able to give it to the group, I suppose. Half-elves/drows could have the skill with only half the effectiveness. Humans, being the wheeling, dealing scoundrels they are, would naturally have the ability to turn up a little charm. Making them seem more pleasing to the group and such. Or, if that seems a little overpowered, how about being limited to using it only while in a group. "Bob flashes a smile and really turns on the charm." Though, a smiling human anti-paladin would probably creep me out.

Not sure if this has ever been suggested, but just been at the back of my mind for a while.