Excellent on-line site selection tools are:
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. Click Here to demo the application for selected sites.
pvPlanner is a simulation tool for effective and intuitive planning of a PV system at any location. The tool works with high-resolution climate and geographic data, and allows you to compare the energy yield from various PV technology options (e.g. crystalline vs amorphous silicon modules) for any selected ground or rooftop installation. Click Here to to demo the application for selected sites.