The short answer is a lot of different kinds of organizations. You can read more about some examples below, and exploring the resources at O*NETOnline can also be a helpful place to start.

Local Government: city/county planning department, insect control, municipal water districts, etc. 

State Agencies such as Oregon Department of Fish and Wildlife will vary from state to state, but most states have agencies in charge of environmental regulation, forests and wildlife, parks, etc 

Federal Agencies such as the Environmental Protection Agency, US Fish and Wildlife Service, National Oceanographic and Atmospheric Administration (NOAA), National Park Service, US Forest Service, Bureau of Land Management, US Department of Agriculture, US Geological Survey, etc. 

Non-Governmental Organizations (NGOs): Nature Conservancy, Sierra Club, Natural Resources Defense Council, Environmental Defense Funds, etc. 

Private Companies: Environmental consulting firms, timber companies, pharmaceutical companies, medical-services laboratories, hospitals, biotechnology companies, zoos, etc. 

Academic Institutions such as universities and community colleges, as well as university run facilities such as institutes and field stations. 

For more information about any of these types of work see the U.S. Bureau of Labor Statistics Occupational Outlook Handbook