IsGroundedFrom http://answers.unity3d.com/questions/196381/how-do-i-check-if-my-rigidbody-player-is-grounded.html
var distToGround: float;
function Start(){
// get the distance to ground
distToGround = collider.bounds.extents.y;
}
function IsGrounded(): boolean {
return Physics.Raycast(transform.position, -Vector3.up, distToGround + 0.1);
}
function Update () {
if (Input.GetKeyDown(KeyCode.Space) && IsGrounded()){
rigidbody.velocity.y = jumpSpeed;
}