C++ 128/256-bit fixed size integer types

I was wondering if any fellow SO's could recommend a good light-weight fixed size integer type (128-bit or even 256-bit, possibly even template parametrized) library.

I've had a look at GMP and co, they care great, yet are a bit too large for my purposes, I'm interested in simple header only solutions at this point. Performance is important and the target architecture will be x86 and x86-64, also a reasonable license (aka nothing GPL or LGPL).