As of 2015, some sources for free fireplace mantel plans includes PlansPin.com and This Old House. All of these plans are available online with links to the detailed plans.
At PlansPin.com includes fireplace mantel plans that are simple and others with decorative touches, such as beveling. There are plans for mantels that fit over a fireplace with an adjacent flat screen television. There are patterns to make mantels from paneling and some from pine boards.
This Old House offers a plan for a wood mantel piece. Builders can hover over the All Steps button to see the work involved and then click on Next Step to find the details of each step.