There is a wealth of curriculum/lesson plan material on the web.

In general, the material posted by education related sources is less elaborate (some might say more achievable). Material offered as part of a broader, subject related site is often quite detailed and ambitious.

It is hard to distinguish between sites that offer off-the-shelf lesson plans, and those that provide lists of links to recommended subject resources from which an educator could construct a unique lesson. Many sites simply mix both types of resources.

Awesome Library    (http://www.awesomelibrary.org/)

The Awesome Library organizes the Web with 30,000 carefully reviewed resources, including the top 5 percent in education. Links not just to lesson plans and curriculum building ideas but also background resources for reading and images.
(Last checked 8/18/08)

AT&T Knowledge Network Explorer    (http://www.kn.att.com/)

Called Knowledge Network Explorer, this is a great general site for those interested in integrating technology. It aims to help schools and libraries acquire and effectively use Internet and videoconferencing technology via online lessons and activities.
(Last checked 8/18/08)

Ben's Guide to U.S. Government for Kids     (http://bensguide.gpo.gov/)

This site provides learning tools for K-12 students, parents, and teachers. The resources teach how our government works, the use of the primary source materials of GPO Access, and how one can use GPO Access to carry out their civic responsibilities. Ben's Guide also provides a locator service to U.S. Government web sites developed for kids. The site is in some disrepair, with dead links and poorly maintained pages.
(Last checked 8/18/08)

CyberBee   (http://www.cyberbee.com/)

CyberBee is Linda Joseph's cartoon character adopted to guide teachers on Internet adventures. A busy little bumblebee zooming around the Internet scouting out curriculum treasures. Many excellent suggestions for web based classroom activities available here.
(Last checked 8/18/08)

CybersmartCurriculum   (http://www.cybersmartcurriculum.org/home/ )

First-of-its-kind K-8 Curriculum co-published with Macmillan/McGraw-Hill and available free to educators. Offers original standards-based lesson plans and student activity sheets, non-sequential for flexible implementation. Empowers children to use the Internet safely, responsibly and effectively.
(Last checked 8/18/08)

Education Place     (http://www.eduplace.com/)

Education Place from Houghton Mifflin has a great deal of information to assist teachers. Look under Activity Search for lesson plans and similar materials by subject and level.
(Last checked 8/18/08)

Education World     (http://www.education-world.com/)
Education World's Lesson Planning Center     (http://www.education-world.com/a_lesson/)

This is an inclusive site, it tries to list everything. It also includes reviews of some sites as a way to provide information about quality. Even though it is huge, the organization is transparent and navigation to various spots of interest is straightforward. The advertising is a bit aggressive.
(Last checked 8/18/08)

Educator's Reference Desk Lesson Plans     (http://www.eduref.org/Virtual/Lessons/index.shtml)

A no-frills but exceptionally well organized database of ready to use lesson plans, most submitted by teachers. Listings include subject and grade level. Plans are reviewed and selected, but there are still some typos and uninspired plans.
(Last checked 8/18/08)

Gateway to 21^st Century Skills     (http://www.thegateway.org/)

GEM is a 700 member Consortium effort to provide educators with quick and easy access to thousands of educational resources found on various federal, state, university, non-profit, and commercial Internet sites. Principal sponsor NEA (National Education Association).
(Last checked 8/18/08)

Internet Projects Registry     (http://www.gsn.org/GSH/pr/index.cfm)

A clearinghouse for collaborative projects from across the globe - projects hosted by site sponsor Global SchoolNet Foundation, other reputable organizations, and partner projects conducted by teachers worldwide.
(Last checked 8/18/08)

Kathy Schrock's Guide for Educators   http://discoveryschool.com/schrockguide/

Kathy Schrock's Guide for Educators, sponsored by Discovery School, is a categorized list of sites useful for enhancing curriculum and professional growth. It is updated often to include the best sites for teaching and learning.
(Last checked 8/18/08)

learnersonline    (http://www.learnersonline.com/weekly/index.htm)

learnersonline features links to curriculum aids for K-12 educators. Arrangement is by subject only, with suggested grade levels as part of each description. Although there are plans listed in most subject areas, the strength of this site is the emphasis on current events. There is also a tie-in to the Newspapers in Education program.
(Last checked 8/18/08)

Lesson Plans Library    (http://school.discovery.com/lessonplans/)

Part of the DiscoverySchool.com site, a wholly owned subsidiary of Discovery Communications. The Lesson Plans Library offers hundreds of original lesson plans, all written by teachers for teachers. Use the pull-down menus to browse by subject or grade.
(Last checked 8/18/08)

Merlot     (http://www.merlot.org/merlot/index.htm)

A searchable collection of peer reviewed, higher education, online learning materials created by registered members. It also offers way to interact with an online community of higher education peers.
(Last checked 8/18/08)

Online Instructional Resources    (http://fod.msu.edu/OIR/)

These diverse teaching and learning resources were reviewed, organized, and annotated
(Last checked 11/2/07)

PBS Teachers     (http://www.pbs.org/teachers/)

Offers more than 1,300 lesson plans, teacher guides and online student activities. Many do not require use of PBS programming. Some are little more than suggestions but the site also contains many pages of detailed plans, arranged by subject and grade level.
(Last checked 8/18/08)

the solutionsite.com    (http://www.thesolutionsite.com/)

Offers K-12 teacher generated lesson plans that emphasize active learning. Consistently high quality lesson plans, organized for efficient retrieval, over a wide range of topics.
(Last checked 8/18/08)

Teachable Moment    (http://www.teachablemoment.org/)

Topical Activities for the K-12 classroom from Educators for Social Responsibility. Offers "timely teaching ideas, lesson plans, essays for discussion, and suggestions for guiding young people as they deal with social conflict inthe world" (Choice, Feb. 2005 p. 1074).
(Last checked 8/18/08)

Verizon Thinkfinity     (http://www.marcopolo-education.org/home.aspx)

Thinkfinity contains more than 55,000 authoritative educational and literacy resources for teachers, students and community programs including standards-based K-12 lesson plans, student materials, interactive tools and web sites. Thinkfinity also includes best-in-class evidence-based literacy resources such as the Life Span Literacy Matrix from the Verizon Literacy Network.
(Last checked 8/18/08)

Yale-New Haven Teachers Institute Curriculum Units     (http://www.cis.yale.edu/ynhti/curriculum/indexes/)

Since 1978, Institute participants have created 143 volumes containing teaching units for English, history, social studies, language, art, math, and science. Units are available for all levels of elementary, middle, and high school.
(Last checked 8/18/08)