GeoSUN highly recommends the following Solargis online prospecting tools.
pvPlanner is a simulation tool for effective and intuitive planning of a PV system at any location. The tool works with high-resolution solar, temperature and geographic data and allows you to compare the energy yield from various PV technologies.
iMaps is an excellent pre-feasibility tool for site identification and comparison as it allows you to see spatial distribution of solar resource in very high detail. Every click on the map shows you the solar potential for the chosen location and allows monthly averages to be downloaded.