Bacteria floating in the ocean use quorum sensing to aggregate into tightly packed, highly organized, and usually slimy communities called biofilms, which attach to hard surfaces such as ship hulls. The U.S. Navy spends more than $100 million every year on fuel to overcome drag on their vessels caused by biofilms. (Photo by Jim Canavan, Woods Hole Oceanographic Institution)
In the human body, bacteria also form biofilms to accomplish things together that they couldn't individually. Plaque on teeth is one example. (Wikipedia Commons)
On the other hand, biofilms also play essential beneficial roles in ecosystems. They provide habitats for creatures in tidal pools, for example, and they decompose and recycle organic material and help keep nutrients circulating in the marine food chain.