What scriptural evidence exists in support of our spirit's pre-mortal existence? Some denominations believe that our spirit existed before we were born on this earth. What scriptural/historical evidence do they use to lay support for their claims?
There is no Scriptural evidence that I find to support that claim. However, I believe that the following scripture does support the idea that not only life, but the Soul begins at conception.


  Genesis 2:7 KJV And the LORD God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. 


Many who do espouse the idea of pre-mortal existence base their assumption on:


  Jeremiah 1:5 KJV Before I formed thee in the belly I knew thee; and before thou camest forth out of the womb I sanctified thee, and I ordained thee a prophet unto the nations.


That Scripture however would appear to validate God's omniscience and not mans pre-mortal existence.