"Thumbs Up" grip

This is one of those little great ideas that people come up with occasionally. According to many very experienced photographers, this little thumb support makes your grip, especially one-handed, of the camera way more secure.
They are not cheap, over $100, but I guess they have to be. They are mostly made for Leica, but I find it interesting that they have a model not only for the Fuji X100, but also one for the Fuji X10, saying a bit about this amazing little camera's status and quality.

You just plug the Thumbs Up into the hot-shoe and tighten it, and there's your thumb rest.

They don't show the X10 model seen on its own, but here is one of the models for Leica:

I often like to walk around with a wrist strap instead of a neck strap, having my camera hanging in my right hand by my side ready to whip up, so I imagine that this accessory would improve the grip in that situation too.