FSMine - Routes

Vataware This contains all the routes performed by VATSIM pilots. You can use the new Top Routes function and specify the origin and destination airports and it will show the most common routes, which will probably be the most correct ones as well.
RouteFinder Contains an up-to-date database of waypoints and has a lot of routes to offer.
VATRoute Not so complete, but you can look for routes by departure or destination. Has few domestic flights.