The site gives excellent descriptions of different clean energy technologies and how they work - photovoltaic solar, geothermal systems, biomass as replacements for fossil fuels, etc. It's more of a technical explanation of how the technologies work, but its written in a way that is easy to read and understand.
They also have a section on clean energy solutions, where I discovered that there is a proposal to build an offshore wind farm off of Cape Cod, which could be a big source of power for the New England region.
Check out the Union of Concerned Scientists website. It is pretty awesome.