Though they reproduce by spores, slime molds are not actually fungi. They're members of the Protist kingdom, along with lots of other hard-to-classify one-celled things.
The slime mold life cycle is like no other. They begin as a giant one-celled, multi-nucleate blob called a 'plasmodium'. Hungry for bacteria, young plasmodia pulse through detritus such as rotting logs and leaves. When they've had their fill, they "fruit" into the millimeter-tall spore-filled reproductive structures pictured here.
I illustrated many of the world’s most common slime molds for Stephenson Lab at the University of Arkansas. Each illustration shows a species at around 40x actual size.