Ecology Fund has buttons you can click for free to donate to help clean the air, help the rainforest and more! And in case I haven't mentioned it here before, we can also help save the forests in Sweden at this website
Also, I have noticed that if you click on this website and other "click for donations" sites you can triple your donating power by doing it in different web browsers! For example if you click on all the charities at http://www.care2.com/click2donate/ using Firefox, you can then open up Internet Explorer and click there all over again, and click again through Netscape.
I click daily using all three browsers and none of the sites recognise that the clicks are coming from the same computer. The websites I do this at every day are:
A Click for the Forest
The Hunger Site and co