This is an on-going list of websites that have a lot of free (or very cheap) resources for homeschoolers to utilize. If you know of one that you think should be added feel free to comment and let me know.
Book Adventure - Free Reading Program for K-8. Kids take quizzes on books they've read and earn points they can redeem for prizes (similar to AR reading program but accessible for everyone).
Shelfari - A great way to visually keep track of all the books your kids read. Tracks books and pages read by year so you can see how you're doing this year compared to previous years.
Starfall - ABCs, Phonics, and Reading
Khan Academy - Over 3,000 instructional videos on math, science, and history.
Math Pickle - Good resource of free math videos plus other information for parents.
MEP Math - A full math curriculum for grades K-9. Has teaching guides, workbooks, etc. All free and printable.
Learning Games / Videos:
Games at Scholastic Books
Magic School Bus Games
Overview of Lapbooks at Squidoo
Lapbook Photos at flickr
Notebook Information Overview
Color Stationery - Can be used as notebook pages.
Notebooking Pictures Group at flickr
Practical Pages - Many free notebooking pages ready to print.
Free Clip Art:
History Clip Art - Many images/clip art of history topics (plus some science).
Open Clip Art Library - Probably the single largest collection of free clip art.
Sweet Clip Art
Various Other Printable Resources:
Dover Books Free Weekly Samples - Sign up by email for free weekly samples across many topics and themes. They give about 20 samples a week.
Head of the Class
Homeschool Share - Lapbooks, notebook pages, unit studies, etc.
HS Launch - Homeschool resource sharing site. Choose "View Files" at the top then search by category to easily find what you're looking for. Has lots of worksheets, lapbooks, and noetbooking pages.
Lined Paper - More lined paper than you could ever need!
List of Places That Offer Discounts to Homeschool Families
Parent Involvement Library - This is a public school resource for parents but has good information about teaching in a hands on way for different subjects that you make be able to get some extra ideas from for use in your homeschool.