If you're using Google Maps and are looking at a route from A to B, you can click any point on the path and drag it to anywhere on the map, and the route will update to pass through the point you selected. Example: http://farm3.staticflickr.com/2835/9667570442_4689c021f7_o.p...
The only downside is you can't search for the place to stop at, you have to find it on the map.