I sure wrestled with which picture to post for this entry. I decided to spare the non-chicken keepers the visual of chicken droppings. However, for a very complete and graphic picto-log of the extremes of poo see this page from the UK called Poultry Pages Forum. The range of characteristics that chicken poo that appears from healthy hens is disparate and extreme. This is due to the fact that chicken have two forms of droppings and diet also greatly affects appearance.
Despite this, is still a good idea to pay attention to changes in droppings. They are an early indicator of bird health. For a list of droppings of concern, this forum page has a long list of descriptors and possible causes.