There are, of course, millions of links with relevance to K-12 education.  Here are a few comprehensive ones, with further links to other sites as well:

 

Bill Timme's Educational Resources:

http://www.nvnet.org/nvhs/edresource/index.html

Kathy Schrock's Guide for Educators:   

http://school.discovery.com/schrockguide/

High School Ace (formerly High School Hub):

http://highschoolace.com/ace/ace.cfm

New Jersey Department of Education:                   

http://www.state.nj.us/education

Teachnology Web Portal for Educators:                                            

http://www.teach-nology.com/

Lesson Plans Page:                                                   

http://www.lessonplanspage.com/

Education World:

http://www.educationworld.com/

EduHound:

http://www.eduhound.com

"Lesson Planz":

http://www.lessonplanz.com

Teachers Network:

http://www.teachnet.org/

McREL Educator Resources:

http://www.mcrel.org/resources/links/hotlinks.asp

NY Times Daily Lesson Plan:

http://www.nytimes.com/learning/teachers/lessons/index.html

PBS TeacherSource:

http://www.pbs.org/teachersource/

Teachnet.com:

http://www.teachnet.com/