Dictyostelium discoideum
D. discoideum is a social amoebae or slime mold commonly found in soil and moist leaf litter where it feeds on other bacteria like Escherichia coli. Due to it's unique life cycle which allows many cells to aggregate and form multicellular fruiting bodies when food is scarce, D. discoideum has become model organisms for studying cell signaling, cytokinesis and cell differentiation in ways not found in other model organisms.
[*] source: http://news.nationalgeographic.com/news/2011/110119-amoebas-smallest-farmers-discodeum-science-nature-bacteria/