While it may come as a surprise to many Westerners, especially those adhering to the 'big three' faiths (Judaism, Christianity and Islam), the most peaceful major religions still practiced today trace their origins eastward.
From a global perspective, Buddhism and Jainism originated next door to the major Western religions, in India. Daoism originated further east, in China. All of these religions can be considered ultimately peaceful, and while some have been used in an attempt to justify war, those searching their scriptures and basic teachings are hard pressed to find any justification of violence.
Daoism
Daoism is very abstract and mystical, but at the same time, isn't steeped in the metaphysical: rather, it puts forward many of its ideas in linguistic puzzles which, while serving to undo unnecessary conceptions, can reveal truths in an ironic way.
Daoism sheds the grandeur of many other religions and adopts a more playful, relaxed philosophical perspective. A lot of Daoism's focus rests on perceptions of balance, demonstrated by the popularized circular figure of yin and yang. Its teachings can be traced back to the "Daodejing," its central text. It also has a close relationship with "I Ching," a book centered around a systematic form of divination.
"Dao," roughly translated, means "way" or "path." Another important term to Daoism is "wu-wei," or "effortless action:" a goal of Daoism is to perform in life in a way that is without effort or difficulty, fitting into daily life with a simplistic perspective, avoiding unnecessary work and avoiding causing unnecessary work for others. To Taoism, there is no point in achieving self-aggrandizing power or fighting with the world: the world is the way it is, and there's almost nothing you can do about it, so deal with it. While kindness is important, happiness is more important than perfection.
The philosophy of Daoism has entered many minds, nagging at them to explore it and write books about it. For an introduction to Daoism, check out "The Tao of Pooh," a short but poigniant work explaining Daoist philosophy through the A. A. Milne's famous characters: notably, Winnie the Pooh is an example of a Daoist. "The Tao of Physics" is a good one for more advanced readers, comparing the puzzling philosophies of Taoism to the even-more-puzzling quantum theory, and finding the two to be remarkably similar.
Buddhism
The Buddha, having considered first a life of luxury, then dedication to asceticism, emerged finally from an enlightening meditation session with a "eureka!" and went on to gain followers and preach about middle path (not too much, not too little), the nature of suffering, and how to relieve it.
Buddhism credits suffering to the mind, and outlines its three causes: avoidance (wanting to get away from something; fear, anger and laziness fit here), attachment (wanting to get closer to something; desire and clinginess fit here), and ignorance (lacking knowledge; immaturity and impulsiveness fit here). These three things, in turn, are centered around the biggest problem: belief in the 'self' or 'sense of "I"' as an actual legitimate thing to protect and appease.
The Buddha says there is no actual "self" - it is a persistent illusion, and dropping it, among other concepts, can lead to revolutionary realizations about living and suffering.
Buddhism seeks to explain why desire to obtain a thing isn't sated by obtaining the thing, and at its core, treats karma not at something attached to the soul (the Buddha preached 'anatman' or 'no soul'), but a predictable system of cause and effect. Enlightenment (and the cessation of suffering) involves full understanding of karma and the absence of the soul.
The eightfold path, which is a guide to attaining enlightenment, shows Buddhism's followers how to minimize suffering not only for themselves, but for other beings, as living things are not fully (only conceptually) separate from one another. Speaking of concepts, Buddhism encourages its followers to let go of attachment to conceptions about the world, which are only useful to a point, and can get in the way of enjoying life and growing enlightened.
Jainism
Jainism is almost anal-retentive in the attention it pays to practicing nonviolence and preventing negative karma. It doesn't support the idea of a conscious creator, but rather, a complex universe guided by specific laws, including karma - that is, the pattern of cause and effect, and the tendency of its results to attach themselves to an eternal soul.
The tradition is dated back to about 900 BCE, but may go back further, and claims everything on Earth to be teeming with life. Negative karma is created, and harm is done, even in eating, walking, digesting, breathing - that is, living. Millenia before the discovery of germs, Jains developed a belief in "nigodas:" tiny living things which are too small to see, but are everywhere. Even harm done to nigodas creates negative karma.
The goal of Jainism is to discover the true nature of the soul and reach 'Moksha' (the liberation of the soul from reincarnation and suffering). This is purportedly achieved by learning discipline, thinking independently, and doing as little harm as possible. Often, those dedicated to Jainism wear simple robes and cover their faces with cloth to prevent harming small creatures by accidentally breathing them in. Brooms are carried to sweep insects out of one's path when walking, so as to avoid stepping on them. Jains avoid eating meat, and even some vegetables, for fear of the harm a more diverse diet may cause to living things.
While negative karma is created even in simple acts, how can a person expect to be able to live without guilt, or at least, relieve the guilt before they die? The religion does appear to be impossibly demanding. Some Jains who are nearing the ends of their lives approach death in a spiritual fast from eating or drinking, thus "undoing" some of the negative karma inescapably gained in life. Fasting until death, in Jainism, is called "Santhara."
By the way, Mahatma Ghandi's extreme peacefulness, asceticism, and fasting can be credited partially to the influence of Jainism. For all the above reasons, Jainism should be considered the most peaceful religion in the world.
Sources:
Buddhanet:
http://www.buddhanet.net
Jain ism at Wikipedia:
http://en.wikipedia.org/wiki/Jainism
Daoism at Wikipedia:
http://en.wikipedia.org/wiki/Daoism