value	The value the user can modify.
position	The position of the handle.
rotation	The rotation of the handle.
size	The size of the handle.
capFunc	The function to use for drawing the handle e.g. Handles.RectangleCap.
snap	The new value after the user has modified it. Note: Use HandleUtility.GetHandleSize where you might want to have constant screen-sized handles.

Make a single-float draggable handle.

This is used to make the center scale handle. The user can click and drag to scale a single float up and down.

Scale Value handle in the scene view with an arrow cap as the handle.

	// Increase/decrease a value just by moving an Arrow Handle
	
	@CustomEditor (ScaleValueModifier)
	class ScaleValue extends Editor {
		function OnSceneGUI () {
	        target.floatVal = 
	        	Handles.ScaleValueHandle(target.floatVal,
	 					target.transform.position,
	 					Quaternion.identity,
	 					30,
	 					Handles.ArrowCap,
	 					1);      							
	        if (GUI.changed)
	            EditorUtility.SetDirty (target);
	    }
	}

And the script attached to this Handle:

	// ScaleValueModifier.js
	// Usage: Place this script on the Game Object 
	// you want to modify "floatVal" by dragging the Arrow Handle
	
	@script ExecuteInEditMode()
	
	var floatVal : float = 1;
	
	function Update() {
		Debug.Log("Value Modified: " + floatVal);
	}