mostly biology-based
Ecological Society of America
Missouri Dept of Conservation
Rocky Mountain Biological Laboratory
Summer Research Program in Ecology Application Process (Harvard Forest)
Society for Conservation Biology (SCB)
Texas A&M University Wildlife and Fisheries [People] (listings across the field and institutions)

multiple fields
Idealist.org - Imagine. Connect. Act.
Americorps (emphasis on environment, health, community development, and hunger)
Student Conservation Association (conservation corps and leading HS crews)
Moving Ideas
MonsterTRAK GreenCareers
Missouri Dept of Natural Resources
National Wildlife Federation
Smithsonian Natural History Museum
USA.gov: The U.S. Government's Official Web Portal (THE website for federal govt. jobs)
Teach for America

agriculture
Center for Prairie Studies (with Environmental, Historical, and Public Affairs opportunities)
WWOOF - World Wide Opportunities on Organic Farms
 

 
environmental-based
Association for the Advancement of Sustainability in Higher Ed (sign up for the free, weekly AASHE Bulletin for loads of resources/info, inc. jobs)
Smithsonian Environmental Research
NREL: Employment Home Page (National Renewable Energy Laboratory)
The White House Council on Environmental Quality
Care2 (variety of job listings, from energy to ecology)
UN Environment Program Internship
Appropriate Infrastructure Development Group (international sustainable development)
Environment America (grassroots organizing)

health-based
The Global Health Corps