Matryoshka: a font in a font in a font
Matryoshka is a display layering type family which is inspired by the Russian wooden doll. The family contains eight different weights from XXS (thin) to XXL (fat) + Pregnant (all in one). It was designed by Peter Brugger in 2009/10. The design is based on an elaborate and complex grid, so each font fits perfectly into the other.
You can download it for free on Volcano Type.