The sixth chakra is known as the Brow Chakra, Third Eye, or Ajna Chakra. It is located between and above the eyes. It doesn't have an associated element as do the lower chakras, but, as the Vishuddha Chakra is connected to sound, Ajna Chakra is connected to sight. Visual consciousness, intuition, dreams, and perception are governed by the Third Eye. The eyes as well as the pineal gland in the brain are the domain of Ajna Chakra. If something is "seen" in the mind's eye, it is the Third Eye that is seeing. In the subtle body, there are considered to be three main channels for energy (as well as countless smaller ones, known as meridians or nadis), known as Ida (left side, yin energy), Pingala (right side, yang energy), and Shushumna (central, main channel). At the Third Eye, these channels converge. For a basic intro to the subtle body, check out my post here. Trataka (steady gazing), meditation, and eye exercises are good ways to stimulate the Ajna Chakra.
Special thanks as always to the groundbreaking work of Anodea Judith for chakra information.