Earth Sciences

Cyber-Sierra: resource conservation and related fields.
Earthworks: numerous links by field; post "short" resume for free.  National and international
positions, majority at colleges and universities.
Geological Society of America: coordinates a wide variety of educational and career-enhancement opportunities.
GEOJob Source: lots of listings in the GIS fields.
Geotimes: career positions and student opportunities. 
GIS Jobs Clearinghouse: positions listed in the USA and in Canada.
InfoMine: positions within  the mining/related industry.
Oil and Gas On-Line Career Center: positions of all types within the oil and gas industry.
SCA (Student Conservation Association): listing of SCA conservation internships.
Water Resources: information and job listings for water resource openings. 
Wetlands: many positions relevant to careers in wetlands ecology.

Environmental

ARS: science positions available with USDA - AgResearch
Cyber-Sierra: resource conservation and related fields.
Earthworks: numerous links by field; post "short" resume for free.  National and international positions, majority at colleges and universities.
Environmental Careers World Job Bank: Biology and Ecology jobs.
Environmental Careers Organization
Green for New England: positions listed include internship and career-related (click on "getting involved")
National Environmental Directory: directory of more than 13,000 organizations in the United States concerned with environmental issues and environmental education.
National Fish and Wildlife Foundation: information on projects included; also partnerships with other conservation-minded organizations.
National Institute for the Environment: includes listings of positions in the environment and other resources.
Outdoor Action Guide to Environmental Careers: information on wilderness/outdoor environmental jobs.
Student Conservation Association (SCA): resource for environmental positions, both internships and employment.
SUWS Adolescent and Youth Programs: outdoor experiential therapeutic program.

Health-related

All the Virology on the WWW: comprehensive list of virology companies.
America's Health Care Source: 1000's of positions in fields from clerical to physician. "Internet's #1 Health Care Career Site.
BioOnline Career Center: extensive database of the top employers in life science fields.
BioRole.com:  Staffing & Recruitment for the Biopharmaceutical industry.
Center for Disease Control: Employment opportunities    
Health Career Web: jobs for Healthcare professionals.
HealthJobsite.com: Includes more than 300 healthcare related companies.
HospitalJobs Online: the Healthcare Job Connection.
Medsearch America: search jobs, search resumes, view profiles of employers, and search career resources.
Pharmaceutical Jobs: list of jobs within the pharmaceutical industry, including medical, sales, and international positions.

Life Sciences

American Association for the Advancement of Science: resources, information, and job postings for many science fields including newer areas such as pharmaceutical/biotech corporate management, entrepreneurship, science journalism, regulatory affairs, science policy, and bioinformatics.
Aquanet: very good site for marine biology and marine science.
ARS: science positions with the USDA-Ag Research
Audubon Online: information on the society, as well as ways to help.
Biological Resources Division of USGS: numerous positions through various sites for environmental positions.
BioOnline: resources of biotechnology companies, biotechnology centers, research and academic institutions, industry suppliers, government agencies, and non-profit special interest groups.
Biospace: resources and information in the biotechnology and pharmaceutical industry.
Biotech Directory: Life Sciences resources on the Internet.
Biotech on the Net: Life on the Net (includes career center).
BMENet: a biomedical engineering resource.
Employment Opps w/Game, Fish, & Parks: with the state of South Dakota.
Environmental Careers World Job Bank: Biology and Ecology jobs.
Genentech. Inc.: biotechnology company job postings.
Job Sites: University of Massachusetts Dept. of Natural Resources Conservation Links.
Limnology and Oceanography: position announcements.
Massachusetts Biotechnology Council
SCA (Student Conservation Association): national listing of SCA conservation internships.
Sciencejobs.com: for US bioscientists.
The WWW Virtual Library (Bioscience): World Wide Web sites relevant to Bioscience.

Physical Sciences

American Chemical Society: also with job listings.
American Institute of Physics: daily updated list of job opportunities.
Associated Western Universities Incorporated: Research Fellowships for undergraduates, graduates, and faculty in science and engineering.
eLabRat: numerous positions in chemistry and other sciences.
Physics Job Announcements: job announcements by thread. 

Social Sciences 

Archaeology on the Net: fieldwork opportunities
Berkeley Job Bank: jobs in media, including newspapers, photojournalism, television.
BlueLine: jobs in law enforcement or fire fighting. 
CWeb Law Enforcement: many law enforcement positions.
GeoCities: includes a number of sites announcing linguist positions.
Job Openings for Economists: a listing of job openings published by the American Economic Association.
Jobs4Police.com
lawenforcementjobs.com: (subscription service) law enforcement and private security jobs listing.
LibJobs: posts both library and museum jobs.
MERC: Museum Employment Resource Center
Minnesota BCA: Bureau of Criminal Apprehension Forensic Science Laboratory Epstein-Rhoads Intern Program.  Internship information for those wishing to entering the field of forensic science.
National Building Museum: positions of interest to historians/museum job seekers.
National Employment Listing Service: several hundred criminal justice jobs, including the fields of law enforcement, courts, and correction.
National Writers Union: lots of positions in writing fields.
PoliceEmployment.com: information and job listings on local and federal police careers.
Psychology/Sociology links:  many links from Ohio Northern University.
Public Relations Society of America: includes positions in Public Relations.
Social Sciences: sites and resources pertaining to the social sciences (via "The Riley Guide").
Social Work and Social Services Jobs On-Line: job database; career resources; and other areas of interest to those in social work.
Society of Architectural Historians
SocioWeb: lots of information on social work and related positions.
The WWW Virtual Library (Sociology): World Wide Web sites relevant to Sociology.

Multiple Science Fields

RS: science positions available with the USDA-Ag Research
Cyber-Sierra: resource conservation and related fields.
Earthworks: numerous links by field; post "short" resume for free.  National and international positions, majority at colleges and universities.
ECEA Jobs Online:  post your resume online to see jobs in the Environmental, Construction, Engineering and Architectural fields.
Environmental Careers World Job Bank: Biology and Ecology jobs.
Essentials: information on approximately 60 non-profit organizations.
National Oceanographic Data Center/US National Oceanic and Atmospheric Admin.
SCA (Student Conservation Association): a national listing of SCA conservation internships.
Science's Next Wave: the scientist's source for career and job hunting information.
Temple University: resources in biology, chemistry, environmental studies.