To get the best ideas and resources for any content area visit organization or association websites.  Also, visit professional journals online and find a vast array of information as well.  Some journal sites require a subscription, but there is still much information available without subscription including links to other online resources and recommendations for print materials.
History
National Center for History in the Schools
http://www.sscnet.ucla.edu/nchs/
Music
The National Association for Music Education
http://www.menc.org/
Family and Consumer Sciences
American Association of Family and Consumer Sciences
http://www.aafcs.org/fcs/index.html
Art
National Art Education Association
http://www.naea-reston.org/index.html
Social Studies
National Council for the Social Studies
http://www.ncss.org/
Foreign Language
American Council on the Teaching of Foreign Languages
http://www.actfl.org/
National Council on Economic Education
http://www.ncee.net/
Middle School
National Middle School Association
http://www.nmsa.org/
Math- K-12
National Council of Teachers of Mathematics
http://www.nctm.org/
Science K-12
National Science Teachers Association
http://www.nsta.org/
Science Education Foundation of Indiana, Inc.
http://www.sefi.org/
Language Arts- K-12
National Council of Teachers of English
http://www.ncte.org/
International Reading Association
http://www.reading.org/
General
National Education Association
http://www.nea.org/
Special Education
Special Education Association
http://specialed.freeyellow.com/
Council for Exceptional Children
http://www.cec.sped.org/
LD Online
http://www.ldonline.com
Natl. Council for Agriculture Educ.
http://www.agedhq.org/
National Assoc. of Agricultural Educators
http://www.naae.org/