Many people have raised very interesting ideas here.
The biggest issue is that as soon as you start taking ads and accepting money for services then you start having to make value judgments and everyone has different value sets.
With any green advertising you'll soon get into an argument as to whether a product is genuinely green or just a greenwashed product.
C Robb W's Prius vs SUV example springs to mind. Would you take an ad from Toyota? And where do you draw the line? Even a Prius is not "green", it is just less damaging.
In ad space you also need to consider things from the advertiser's side. Consider the Totota example again: Perhaps 99.999% of celsias readers are aware of Prius and already track it through other means (TV ads, press etc), so what advantage does Toyota get in placing an ad in celsias?
Once you start taking ad money, particularly for larger corps, you risk alienation of your core membership.
But then again, many of the organisations on the website could already be viewed as grennwashers rather than really green.
I think Danny W's ideas on donations are good. Paypal supports a "donate" service that allows you to collect small payments. But then again, depending on your values you might be anti-eBay (who own Paypal).
If nothing else, donations give you great feedback as to the viability of the website: How much do the readership value the service?
Written in October 2008