I know there are a lot of sea turtle volunteer programs along the Mediterranean Coast -- you can find links to these on
www.seaturtle.org/mtn/jobs -- some may even pay. They also have opportunities in Central/Latin America and SEA.
Also try:
Earthwatch Institute (a little pricey, but great hands-on experience)
www.Idealist.org
www.blue-world.org
http://sfo.pl/eceat
http://atschool.eduweb.co.uk/ecoliza/
http://www.clcp.ro/index.htm
http://www.sabranenque.com/index.htm
Depending on your interests, I think you could find volunteer opportunities almost anywhere!
