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.

Is wear a nice hat. Like you suggested, there is nothing inherently special about humans. The flipside is that there is nothing really wrong with them, either. If anything, their inherent is the ability to benefit from items that improve stats.

I can walk somewhere and find a piece of equipment that improves my charisma faster than I can re-invoke my dark-elf's faerie fire.

While I am sure charisma is important for many reasons, bards and conjurers would benefit most. Bards already have 'beautify,' and conjurers are just nasty. They can safely teleport or even tesseract and find themselves a hat before I can walk there and get it.