The Yin Yang symbol is more influenced by eastern spirituality than contemporary Pagan or Wicca, but it does bear mentioning. The Yin Yang can be found all over the place, and is perhaps one of the most commonly recognized symbols. It represents balance - the polarity of all things. The black and white parts are equal, and each surrounds a dot of the opposite color, showing that there is balance and harmony within the universe's forces. It is the balance between light and dark, a connection between two opposing forces.
Sometimes the white portion appears at the top, and other times it is the black. Originally believed to be a Chinese symbol, the Yin Yang is also a Buddhist representation of the cycle of rebirth, and of Nirvana itself. In Taoism, it is known as the Taiji, and symbolizes Tao itself.
Although this symbol is traditionally Asian, similar images have been found in the shield patterns of Roman centurions, dated back to about 430 c.e. There is no scholarly evidence as to a connection between these images and the ones found in the Eastern world.
The Yin Yang might be a good symbol to invoke in rituals calling for balance and harmony. If you seek polarity in your life, or are on a quest for spiritual rebirth, consider using the Yin Yang as a guide. In some teachings, the Yin and Yang are described as a mountain and a valley -- as the sun climbs over the mountain, the shady valley is illuminated, while the opposite face of the mountain loses light. Visualize the shift in sunlight, and as you watch the light and dark exchange places, what was once hidden will be revealed.