My apologies if someone's already mentioned this- couldn't find a post on it.

RETscreen International is an organization that seeks to build the capacity of planners, decision-makers and industry to implement renewable energy and energy efficiency projects. There website provides a lot of information on renewable energy projects, along with free software for feasibility studies and project planning. Sadly it's currently MS Excel dependent, but they're working on a web-based platform independent version.

www.retscreen.net/