Raycast question! 3rd person shooter

How can I set the Raycast direction to the center of my screen? (3rd person shooter)

If you want your character (in 3D) to shoot at the center of your screen, then you want to convert viewport or screen space coordinates to world space coordinates.

ie, you want view or screen coordinates to translate into some world space coordinate that you can then aim your gun at in 3D.

See Camera.ScreenToWorldPoint() and/or Camera.ViewportToWorldPoint() for this.

Check out the docs here: http://unity3d.com/support/documentation/ScriptReference/Camera.ViewportToWorldPoint.html

If you want normalized 0-1 viewport coordinates, use the viewport function, otherwise use screen one if you prefer screen (pixel) coordinates instead.