Mycelium

Mycelium is only found natively in Mushroom Land Biomes.

Mycelium acts like grass in a number of ways:
 * When harvested, it will yield one dirt block (and therefore can only be acquired via inventory hacks, or SMP commands from an op).
 * It slowly spreads out to cover bare dirt blocks. (Because Mycelium is only found in Mushroom Land biomes, this and using pistons are the only "legitimate" ways to grow it outside the Mushroom Land biome - provide it a dirt path to grow into.)
 * It grows faster when light is near, contrary to the habits of real fungi.
 * When covered in snow, the sides will use the same texture as snow-covered grass.

There are a few differences, though:
 * Saplings and Flowers can't be placed on Mycelium, but Mycelium can spread beneath them, which causes them to drop as items from the spot.
 * Mycelium is also the only block on which mushrooms can stay when an adjacent block is updated despite a light level that is too high for the Mushroom to remain.
 * Unlike dirt or grass blocks, Mycelium cannot be tilled with a Hoe.
 * The tops of Mycelium blocks have a particle effect which look like tiny spores being released constantly from the surface. This effect continues even if the Mycelium block is covered in snow.

Some other points of interest:
 * Mycelium and grass do not replace one another.