When applied to a game object causes the object to billboard to the camera.

public class CameraBillboard : MonoBehaviour
{
    // the camera the the game object will be bill boarded to.
    public Camera m_Camera;
    // if true the game object will be positioned just in front of the cameras near clip plane
    public bool PositionInFrontOfCamera;
    // the offset to position the object when PositionInFrontOfCamera is true
    public float Offset = 0.001f;


    void Awake()
    {
        // if no camera has been specified just use main camera
        if (m_Camera == null) m_Camera = Camera.main;
    }

    void Update()
    {
        // get forward vector of the camera and normalize it
        var vec = m_Camera.transform.forward;
        vec.Normalize();

        // set the position of the game object just inside the cameras near clipping plane so it blocks the camera view
        if (this.PositionInFrontOfCamera) this.transform.position = m_Camera.transform.position + (vec * (m_Camera.nearClipPlane + this.Offset));
        
        // orient the game object to look at the camera
        this.transform.LookAt(this.transform.position + m_Camera.transform.rotation * Vector3.back, m_Camera.transform.rotation * Vector3.up);
    }
}