NEXCESS.NET is 'green' through purchasing green power certificates, however they have a stated goal of being 100% renewable energy powered by December 2009, and that today they are 25% renewable energy powered.
I suppose by being 'greenest host' they are the most green of web hosting operations? Well, rather than rely on green power credits their hosting facilities are in a solar powered data center just north of San Diego. As they say, rather than create the dirty power in the first place and do something else to offset the dirty power, they simply use green power.
They are 'green' through having conducted an audit of all their hosting facilities and purchasing green energy tags to match their energy use. They also have a commitment to social responsibility, social change, use of telecommuting, use of open source software, etc. They offer free web hosting to US based, registered non-profit progressive organizations whose primary focus is environment, grass roots democracy, peace, social/economic justice or human rights.