We don't usually offer consumer advice here, but in reference to a question I just got asked, I thought I'd say a little about Green Electricity Tariffs. I'm afraid this only applies to the UK, so if you are in Durkadurkastan or Molvania, you can stop reading.
So, er, yes. You can sign up to buy green electricity from a number of providers. For example, EDF offer a green tariff which doesn't actually generate any green electricity at all. They simply buy green electricity the Government obliges them to (under the Renewables Obligation) then sell it to you. But double the extra you pay on the EDF Green tarrif goes into a fund which they then give to community projects which support renewables.
npower offer a tarrif called juice which doesn't cost any extra (which is fair enough, because like all other suppliers, to renewable obligation requires them to buy renewable electricity) and for every juice customer they invest £10 in renewables research (hardly a staggeringly large amount but every little helps...)
Meanwhile good energy has a 100% renewables tarrif, where they retire 5% more ROCs than they have to. But that doesn't explain what they do with the other 89.5% of their ROCs (the ones they don't retire). One advantage of this tarrif is that you're paying money direct to a company that doesn't generate any electricity from coal, gas or nuclear. But is it really 100% renewable if they can sell ROCs?
In summary, you should probably be on one of these tariffs but I don't which one. These people might be able to help http://www.greenelectricity.org/