The Ouija board was invented as a toy in the late 19th century. As a kid, I played with one several times without any ill effects whatsoever.
The Ouija board is just a fun party toy. There may be a way to turn it to legitimate spiritual uses, if you create a proper ritual and have the right intent, but don't be fooled or afraid of just using it normally. I guarantee you that it will not open a doorway for demons or evil spirits - that takes a lot of effort and a lot of work, not a mass-produced Hasbro board game!
The results you get from the board are caused by something called the ideomotor effect: small, unconscious muscle movements. These are what cause the planchette to move. This has been proven by studies in which participants were asked to use a Ouija board, and blindfolded; then, without their knowledge, the board was turned upside down. When they began to use the board, the planchette moved to where the letters would be if the board had been right-side up... even though, if the spirits were causing it, they would know it was upside-down and direct the planchette toward where the letters really were.
Thus, it's clear it was really the participants moving it - but without knowing it was them. It isn't that people lie or do it on purpose; the ideomotor effect is unconscious, so you can be moving the planchette but really believe something else is doing it.
You can try this experiment on your friends yourself - just make sure they don't know you've turned the board around.
The idea behind the Ouija board - of having spirits direct answers by use of material objects - has been used legitimately in ancient China. However, the Chinese had a much different, and much more complex, system that enabled them to overcome the problem of the ideomotor effect. For more information, look here: http://en.wikipedia.org/wiki/Fuji_%28planchette_writing%29