This one is pretty neat: biologists at UC San Diego created a living "neon signs" made from glowing bacteria that fluoresce in unison:
In order to create the light they needed to attach a fluorescent protein to the biological clocks of bacteria and then synchronise the body clocks of the bacteria within the colony so that they glowed on and off in unison. The team created signs that spelled out "UC SD".
Using the same technique they used to create these flashing signs, the team also created a simple bacterial sensor capable of detecting arsenic. This sensor would make the cells blink on and off more slowly, indicating the presence of the poison.