I love the pattern on this fungus, the way it almost but not quite blends into the tree trunk. It almost covers a fairly large area where, at some point, a branch was cut off low down the trunk.
Tentative identification via Gardenweb is that it is a Ganoderma applanatum, fascinatingly also called the "artist's mushroom," not, unfortunately for its beauty but for the enduring nature of any bruising to its surface (Apparently some people draw on them. Hang on. Let's see if I can find a more artistic example somewhere... Oh, here's a set, and here, and there are examples here, too. Who'd have thought? I do so love the net as a way of Finding Things Out).
Anyway, the Ganoderma applanatum can also apparently live for dozens of years, which is fascinating. I'll have to keep an eye on this one. Assuming, that is, it is a Ganoderma applanatum and not some other flat fungusy thing. In any case, It's fascinating and I'll keep an eye on it.