I personally requested this feature from Sergei Brin two years ago. OK, I made this up, but I have wished for this feature for at least that long. And voila! You can now click and drag your Google Maps directions to alter the route and corresponding directions.

"Since it's easy to take online driving directions for granted, I wantedto briefly make you aware of a new feature we released today on GoogleMaps. Now when users get directions through Google Maps they cancustomize their route by dragging and dropping the route line over anypreferred highways or roads. The directions instantaneously update,showing new time and mileage estimates as a user drags the route. - Google"

The All Points Blog first reported the feature, full announcement at the Google Lat Long Blog.