position	The position of the current point.
direction	The direction of the sliding.
size	3D size the size of the handle.
drawFunc	The function to call for doing the actual drawing - by default, it's Handles.ArrowCap, but any function that has the same signature can be used.
snap	The snap value (see Handles.SnapValue). Note: Use HandleUtility.GetHandleSize where you might want to have constant screen-sized handles.

Make a 3D slider.

This will draw a 3D-draggable handle on the screen. The handle is constrained to sliding along a direction vector in 3D space.

Slider handle in the Scene View.

	// Simple script that creates a Magenta Slide Handle that 
	// points to (0,0,0) nomatter where the target GameObject is located.
	
	@CustomEditor (Slide)
	class SliderHandleJS extends Editor {
		function OnSceneGUI () {
			Handles.color = Color.magenta;
	        target.vectorPoint = Handles.Slider (target.transform.position, 
	      						Vector3.zero - target.transform.position);
	        if (GUI.changed)
	            EditorUtility.SetDirty (target);
	    }
	}

And the script attached to this Handle:

	// Usage: Place this script on the Game Object you want to use the
	// editor-created slide handle.
	
	@script ExecuteInEditMode()
	
	var vectorPoint : Vector3 = Vector3(0,0,0);
	
	function Update() {
		Debug.Log("Looking at: " + vectorPoint);
	}