Actually there are two that I use almost every week. I always try to carry
1. Listen to the Bible in One Hour
I always have some on hand. When I witness to someone I give a copy to him or her. It is a really nice condensed (60 minute) recording of the Bible. It is all scripture and really powerful. I think when we plant the seed, it is important to give it 'food'. What better food than God's word? And, it is something that the receiver perceives as valuable - so they are thankful for getting it.
It is a free access CD, meaning you can copy it and give it away. The church that my wife and I attend have made copies with our church name on the front. That helps people that I share it with find our church.
2. The CD "Can Science Prove There is a God" This is the incredible testimony of how Hugh Ross, an astrophysicist found salvation through Jesus. It is engaging and deals with many of the issues people deal with today. We live in a 'post Christian society'. Many people think that science and faith are not compatible. Hugh Ross discovered for himself the truth in the Bible. As a scientist, he can share in ways that dispel the thought that Christianity is not compatible with how the universe works.
My church has also made copies of this CD since it is can be copied for ministry purposes.
I recommend everyone be prepared with tools! What are your favorite tools?