In Unity, how can I pass values from one script to another?

There are several ways to achieve this.

If you want the speed variable controlled by a component which is attached to a GameObject MyObject

public class SpeedController : MonoBehaviour
    public float speed;
    // maybe you want restrict this to have read access, then you should use a property instead

In other classes you can do:

GameObject go = GameObject.Find ("MyObject");
SpeedController speedController = go.GetComponent <SpeedController> ();
float courrentSpeed = speedController.speed;

Take care that there is one object named MyObject only otherwise things get messed up.

Alternatively you can define a SpeedController member in every class that needs access to speed and set a reference via drag and drop in Unity editor. You save the lookup then but of course this is pretty inconvenient if needed in many classes.

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Another way is to create a singleton which holds the speed variable and have:

public class MyGlobalSpeedController {
    private static MyGlobalSpeedController instance = null;
    public static MyGlobalSpeedController SharedInstance {
        get {
            if (instance == null) {
                instance = new MyGlobalSpeedController ();
            }
            return instance;
        }
    }
    public float speed;
}   

So all classes can access this:

float currentSpeed = MyGlobalSpeedController.SharedInstance.speed

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As Jan Dvorak stated in comments section:

public class SpeedController : MonoBehaviour
    public static float speed;

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[Update]
Thanks to Jerdak. Yes Component.SendMessage should be definitely on the list:

go.SendMessage("GetFallingSpeed");

Again you need to have a reference to go like described in the first solution.

There are even more solutions to this problem. If you are thinking of game objects that are active in all scenes, you should have a look at Unity singleton manager classes