Here is a link to the value chart, it is a bit easier to navigate than the one copied above. It was developed, I believe, primarily by the Dallas Cowboys front office many years ago when Gil Brandt was still there. Most teams follow it pretty closely so it is a handy link to reference when people start throwing out wild trade rumors.
NFL Draft Value Chart
Edit: Ok, Big C beat me again in the midst of my typing apparently ... apologies for the duplicate