Vestigiality

Some of the most compelling evidence for evolution comes in the form of organs and limbs that have lost their original fuction to some degree. These are known as vestiges, or vestigial organs. A common misconception is that a vestigial organs is required to have no function at all. This is not the case. The requirement for something to be considered a vestige is merely that it does not function as it was once intended to. A good example would be the wings of flightless birds, which are obviously not used for flight but still serve some purpose. The wings of an ostrich are used to balance while running, and are an important part of courtship displays. The human coccyx (aka. tailbone) no longer serves as the base of a tail, but is an anchor point for many muscles used for shitting in the groinal area.
The Talk Origins database has a fairly comprehensive section on vestigiality if you're interested to learn more.