A spiritist or fortune teller is not a Godly person, according to Leviticus. However, there is ample evidence from the Bible that there were Godly people who did indeed see things like angels, demons, spirits, and prophesied from God. <br />
I don't know enough about the subject to say whether opening the third eye is a sin or not. I would venture to guess that it all has to do with WHY you want to do it. Is it to gain greater communion with God or just to follow your own wishes and wants?
Everybody sin's... pick and chose yours. Eve's sin was eating from the knowledge of Good and Evil... I did exercises to try and open my third eye and I started having some pretty funky dreams, in one, I visited the river styx... and in another I saw a human that had been skinned alive...and many more. They werent normal dreams, they were REAL... its like we want to see more, but we also want to see what we want to see...and the two may not be the same thing.
It could be very dangerous too, without proper authorities to guide you. There are many forces that are not of God that can take advantage if you're unprepared.
Make one with a knife...
I'm a Christian, and I'm working on opening my third eye. <br />
I've done research on this, read the whole Bible and searched through it. From my understanding, as long as you remember who your God is, and keep him in mind, it isn't a sin. Don't go to other psychics and mediums who aren't Christians though for guidance, believe in yourself and pray you can do it alone. I'm almost there, you can do it too!