If you want to draw a route over roads, you can use the GoogleMapsRoutePlanner by First Consulting.
Another alternative would be GoogleMaps Customer Marker, also created by First Consulting.
Hope this helps!
See Ward's answers. Both widgets can be used. If you just need to draw a line between to markers, then I would like to suggest the last one:
There is a setting to enable lines between markers and also a setting to not show the markers if the line is shown.