Most of you probably already know about these web sites, but for those who don't, I thought I would post them here.
Here is a good site from google to route you training runs. It will total up the mileage for you, and if you use the automatic selection, it will wind around corners and such when you click your mouse, otherwise you can go manually to draw straight lines. Unfortunately, it doesn't measure trails automatically so you have to wing it there with the manual selection.
Gmaps Pedometer
Here is a site from the USATF which runners have added their running routes in different cities and states. This would come in handy if you are going to travel to a different city and want to do a run there.
America's Running Routes