Concordia International Volunteer Projects (www.concordiavolunteers.org.uk) Short-term community-based projects covering the environment, archaeology and the arts.
European Youth Portal (http://europa.eu/youth) Has various links suggesting volunteering options across Europe. Navigate to the Volunteering page.
Legambiente (http://international.legambiente.it) Offers numerous environmentally focussed volunteering opportunities.
World Wide Opportunities on Organic Farms (www.wwoof.it) For a membership fee of €35 this organisation provides a list of farms looking for volunteer workers.