Fomes fomentarius is a tough perennial polypore that usually becomes hoof-shaped with age; it is found on standing and fallen hardwoods.  Its woody upper surface develops grayish zones, and its brown pore surface features tiny round pores.  When sliced open (no easy task, given its toughness) it is usually composed more of vaguely layered tubes than flesh. 

Along with Piptoporus betulinus, Fomes fomentarius is one of two mushrooms that the Tyrolean Iceman was carrying around 5000 years ago.  He apparently used Fomes fomentarius as tinder. 

Description:

Ecology: Parasitic and saprobic on the wood of hardwoods (especially birches and beech); causing a white rot; growing alone or gregariously; perennial; fairly widely distributed in northern and north-temperate North America

Cap: Up to about 20 cm across; shell-shaped to hoof-shaped; with a dull, woody upper surface that is zoned with gray and brownish gray.