With Google Maps you can calculate the square footage of your roof and build virtual panels. Then RoofRay estimates the output potential of the roof, costs of installation, and ROI. Another section shows existing installations that have already been completed by other people, including specifics of the roof used and total peak power.
Of course, RoofRay does not replace solar experts, but undoubtedly could answer some of your basic questions, and generally it is a quite interesting online service that deserves our attention.
Website: www.roofray.com
Updated 2021: Seems like this site is not working anymore, so you can try: Solar Calculator (solarreviews.com), SUNSEI Solar Design Tool & Google Project Sunroof