Common Split-Gill (Schizophyllum commune)
Schizophyllaceae or Split-Gill Family
 

Small mushroom, a half to 2 inches across. Cap fan-shaped to irregular, white, profusely covered with white hairs, splitting into segments when aged. Stalkless, or with a hint of a short stalk. Technically lacking gills, but having deep folds underneath the cap. The folds are in pairs, and the spore-bearing structures are in between the folds. In dry weather the folds draw together to protect the spores, and in wetter weather they spread. Grows singly or in small clusters on dead wood, logs, branches, even boards. From a distance the mushrooms look to me like small amounts of fresh snow caught in the fissures of the log. Can be found all year, withering in dry weather and waiting for rain.

This is probably the most common mushroom in the world, being well known on every continent except the Antarctica (but maybe if we left some boards there, it might turn up). Occasional in Wildwood, where there is appropriate wood, but more easily noticed in wet weather.

The snowy appearance make this an easy mushroom to identify. The double "gills" clinch the identification.

This species is famous for its ability, in very rare cases, to grow in immunocompromised humans instead of in wood; even producing mushrooms in their sinuses. It is also famous for having over 28,000 sexes; any two different sexes can produce offspring, but two of the same sex are infertile.