TV Fool: This tool allows you to see in a graph of which direction the signal is coming from as well as a signal strength graph.
This is a great way to dynamically see where the signals are coming from and at what strength these channels are.
They also have maps and guides as to how high your antenna needs to be in order to get the signal and what measures you need to use in order to improve your signal.
Another cool feature is the maps they provide to show you the coverage areas of various signals. Using Google maps you will be able to see if the signal you want covers the area you live in.
Below you can see the coverage area map of Global Television: Callsign CIII
Another resource is:
remotecentral: This provides an idea of the signal quality and the relative feedback of users who are getting the signal.
It also shows antenna directions and the strength rating of each antenna. Though not quite as visual or as detailed as tvfool it is still a good resource to find out what channels you should be getting in your area and which direction you should be pointing your antenna to.