Google Maps is very nicely done. If you haven't tried it out, you should. But, there's a couple features it doesn't have that I wish it had. This is where Greasemonkey can really shine. The two ideas: Automatically follow a plotted route and add more address markers to an existing map. Today, someone posted an implementation of a userscript that adds the route following feature to Google Maps. Another Google Maps userscript, this one to generate a map with predefined set of points on it, was also posted.
So, all of this activity inspired me to implement my second idea (since Prakash pleasingly did the first one for me). I present yet another userscript: Google Maps: Add Waypoint. It adds a second text entry box which if an exact address is typed into it, it adds a point on the map. In "local search" mode, there is no description of the point, but in the standard maps mode, the address is shown.