Free Art Lessons, 1-7th grade

http://www.cuchicago.edu/experience/arts/visual-arts/art-lessons/

Free computers for schools (including homeschools)

http://usgovinfo.about.com/library/weekly/aa060901a.htm

PowerMyLearning

This site is an awesome aggregator of learning activities from across the web. Accounts are required, but free.

http://powermylearning.com/

If you never use any other learning site, this one is worth your time.

Current Events magazine for kids

This is an online magazine of current events for kids.

http://www.youngzine.org/

Well worth a look.

Tablet Math: Free Math Videos

TabletMath is actually a subscription service, but they put over 250 math videos on YouTube for homeschoolers, including videos that correlate with popular homeschooling math programs Saxon and Math-U-See.

http://www.youtube.com/user/tabletclass/videos

These are similar to Khan Academy in some ways, but more visually appealing.

Some of the videos correlate with a free math quizzes site: http://homeschoolmathquiz.com/

Free Online Kindergarten/First Grade

Good feedback is coming in about Learning Lynx "Awakening Woods" Curriculum (which I use for my Kindergartners, but could work for first graders, too). Free, online, and complete, including art, music, math, language arts, history, grammar, science, social studies, and more....and it's fun! If you haven't checked it out yet, you can find it here: https://sites.google.com/site/learninglynxclassroom/awakeningwoods/month-1/k-day-01

Reading "motivator" program

Created by Sylvan Learning Centers, Book Adventures is a place you can register (free) and take quizzes on books (K-8th) to compete with other students or win prizes.

http://www.bookadventure.com/Home.aspx

Grammar and Punctuation course

This is a well-done course for middle school through college. It hits mostly on the places most people make errors (so you won't be placing periods at the ends of sentences here). There are lessons and online exercises with instant feedback.

http://www.bristol.ac.uk/arts/exercises/grammar/grammar_tutorial/page_41.htm

K-6 Language arts workbooks

Three levels for each grade, in .pdf format

http://www.macmillanmh.com/languagearts/2001/teacher/teachres/workbooks.html

and for Grammar, literature, and spelling:  http://www.mhschool.com/reading/treasure_workbooks/national.html

Animated Tech lessons

http://activities.macmillanmh.com/Techknowledge/TKMacmillan.php

Some of these are outdated, but some are still very useful.

Engineering Activities

Discover Engineering has some awesome engineering activities (how do you keep a plant nicely watered for a week while you're on vacation? Can you build a tunnel through sand without touching it?

http://www.eweek.org/EngineersWeek/DiscoverE.aspx?ContentID=90

There are even more, including full curricular units, here:
http://www.teachengineering.org/

Media-rich history of railroads and steam trains

http://www.makingthemodernworld.org.uk/stories/the_age_of_the_engineer/01.ST.04/?scene=1

Fun science experiments to do at home

http://www.sciencemuseum.org.uk/educators/classroom_and_homework_resources/resources/~/media/EC513F6B0F28453C8EBF70D6557A2042.ashx

This is a fun booklet of science experiments produced by a science museum in the UK.

Parts of a Computer

http://www.kids-online.net/learn/clickjr/clickjr.html

Free Computer Science Classes

http://www.udacity.com/

Learn to Read Course (for ESL speakers and native speakers)

This requires a free registration. http://www.strivney-english.com/index.asp

Cool interactive world map

You can zoom in and out, write on the map, and find distances between places.

http://www.glencoe.com/sites/common_assets/socialstudies/eow_notwdb_v2/NOTWDB_1.html

Printable graphic organizers

http://www.mhschool.com/resources/graphic_organizers.pdf

Beverly Cleary Website

http://www.beverlycleary.com/

Free Internet Skills and Safety Lessons, K-12

http://cybersmartcurriculum.org/lessonsbygrade/ (this is only available until July 2012)

This includes lesson plans, links to activities, and free printable worksheets in .pdf format.

The updated version is here: http://www.commonsensemedia.org/educators/curriculum/k-5 (this one is available after July 2012 as well)