PreK - 12

Agent K-12: from Education Week & Teacher, "sudden vacancy" listings.
Alaska Teacher Placement: Alaska's statewide clearinghouse for the placement of educators in Alaskan schools districts.
America's Job Bank: tremendous variety of positions listed, including education positions.
Association of Boarding Schools: with employment links.
CalTeach: Set up especially for recruiting teachers in California.
Database of US School Districts: National Center for Ed Statistics.
Department of Defense Education: on military bases.
DoD Schools Address list: Department of Defense schools can be accessed by region.
Education America Network:  One of the country's largest online source of education employment opportunities.
Education Jobs page: jobs in public education, nationwide, concentrating on Midwest.
EDUCAUSE: Job Posting Service: positions related to technology-based information resources.
Employment Resources for Language Teachers: Ohio University site with merit .
ESL Employment: teaching positions worldwide.
ESL International Jobs Board: positions listed by Dave Sperling.
JobHunt: a mega-list of online job-search resources and services.
K-12 Jobs: the bridge connecting teachers, education students, and administrators to public/private k-12 job positions listed in the Internet.  Includes job search, job posting, information about certifications, salaries, job fairs, and associations.
Linguistic Funland's TESL: page for jobs in TESL.
NannyNet: career opportunities as a nanny or in the child care profession.
National Education Employment Network: positions in public schools.
Peace Corps: teaching positions and information about the Peace Corps organization.
SchoolSpring: the Employment Source for Educators  Enter your information once and apply for any job, anywhere! 
schoolSTAFF: information to help educators and future educators find jobs in education. 
Teach for America
REAP: listings for numerous states.
Teach Away: specializes in ESL teaching placements in Asia.  
Teachers Support Network: search for open positions nationwide.
Teachers-Teachers.com: academic placement service, free
University of Kentucky: links to all 50 states and employment in education
USA Employment: free site for national and international teaching jobs.
Yahoo's Education Resources: various resources related to the field of education.

Post-Secondary

Academic Careers online: discover full & part-time, visiting, assistantship, and practical experience teaching, research, and administrative opportunities in the USA, Europe, Asia or anywhere else in the world.
Academic Employment Network: links to teaching and academic professional positions in colleges/universities.
Academic360.com: search by state, by name of two or four year college or university for positions within the institution.
Academic Position Network: faculty, administration, and professional staff positions; graduate assistantships and fellowships; and post-doctoral opportunities available at institutions of higher education around the world.
American Association of Community Colleges: positions in community colleges.
American Universities Online: listing of online universities.
Chronicle of Higher Education: current articles and job listings in higher education. 
American Association for the Advancement of Science: teaching vacancies in the science profession.
Employment Resources for Language Teachers: lots of position announcements, including many in post-secondary schools.
HigherEd Jobs: list of Higher Education positions.
Peace Corps: teaching positions and information about the Peace Corps organization.
Post Docs: information on post doctoral positions.
UniversityJobs.com: gateway to Higher Education careers.

International

Employment Resources for Language Teachers: lots of position announcements, including many in post-secondary schools.
ESL Employment: The ESL/EFL Job & Employment Resource Site
ESL Jobs Offered Bulletin Board: Ohio University site with merit.
Footprints Recruiting Incorporated: successful corporation from Vancouver with positions in Asia and South America; posts from March to September.  
Linguistic Funland's TESL: jobs in TESL.
Peace Corps: teaching positions and information about the Peace Corps organization.

Healthcare

Absolutely Healthcare: 275,000+ healthcare & medical jobs.
American Public Health Association: Public Health Career Mart
Exercisejobs.com: "the online career center for fitness and exercise professionals".
Public Health Employment Connection: Public health jobs worldwide "the only comprehensive source of public health careers worldwide".

Social Service/Human Development

Human Services Jobs: from NationJob
Idealist.org: "Action Without Borders"
Career Network: site devoted solely to Human Service, Social Service, and Non-Profit professionals!
The New Social Worker Career Network: Megalist of links! 
Social Service.com: Job Seeker Services - Search Jobs, Post Your Resume - Free!
Social Work & Social Services Jobs On-line: from George Warren Brown School of Social Work, Washington University