Fresh hardwood mulch is a great substrate for this amazing-looking slime mold. Remarkable in all of its stages, it progresses from a gelatinous yellow stage to darkening crusty stages, all rather alarming in appearance.
We had at least 20 different individuals in various stages of maturity on fresh mulch last weekend.
Sometimes called dog vomit fungus, it's a slime mold (Fuligo septica). This informative article in the Chicago Tribune describes its biology nicely.
An ancient organism, it's not a plant, animal, bacteria, or fungus, but it's well-adapted to doing what it evolved to do: break down organic matter.