Clean Up The World (CUTW) is the name of a U.N.-sanctioned community-based project that was created in 1989 by Ian Kiernan. It started with the clean up of the Sydney Harbor by 40,000 volunteers, and now in 2005, the latest example is the cleaning up of turtle nesting beaches in Malaysia by 3,400 volunteers (they invited 2,000, but more showed up), many of them schoolchildren. "The clean up is not the answer to the whole problem but it is a very good start. What is important is the behavioral change among people that it brings," says Kiernan. The big cleanup happens in more than a hundred cities every year, on the second-to-last weekend of September, and attracts some 35 million volunteers, according to Clean Up The World (we suppose that's a cummulative number of volunteers over the years).
The organization also encourages other actions; for example, their website has a page on composting. From the website, you can navigate to a CUTW event close to you (the USA page is here, the one for France is here, etc).