Complete Curriculum Options

Preschool
Brightly Beaming Resources (PreK-Early Elementary)
First School Preschool (thematic units, Pre-K-K)
Read to Me and ABC (PreK) **Added 7/24/13

Charlotte Mason
Ambleside Online (K-12)
Mater Amabilis (Catholic, K-8)
Higher Up and Further In (AO-inspired, K-12)
Tanglewood School (1-8)
Simply Charlotte Mason Curriculum Guide (K-12)
Milestones Academy (K-10)

Classical
The Classical Curriculum (PreK-12)
Classical Homeschooling (K-12)

Primarily Public Domain Text-based
An Old-Fashioned Education (K-12)
Your Free Homeschool Library (K-12) - While I do not provide a schedule or specific book recommendations, I have gathered a plethora of vintage textbooks and categorized them by grade and subject, making it simple for a teaching parent to choose what they need to teach most of the core subjects.

Primarily Web-Based
Lesson Pathways (K-5)
Easy Peasy All-in-One Homeschool (K-High School)
EZSchool (K-12)
DiscoveryK12 (K-12) *Added 9/6/16*
HippoCampus (covers math, science, social studies, and religion, Middle-High School)
Connexions (High School-Adult)

Flashcard/Worksheet-Based
Free World U (K-12)
Head of the Class (K-5) *Update 7/5/13* This site appears to be closed, but I will leave here in case it returns.

Core Knoweldge-Based
Baltimore Curriculum Project Core Knowledge Lessons (K-5)

Single Year Unit Study
Teaching with God's Heart for the World (Elementary)

School Curriculum Guide
Hillsdale Academy Curriculum  (requires free registration, K-12)
Liberty Academy Online Curriculum Guides (scope and sequence, 3-12)

Reformed
Puritan's Home School Curriculum (K-12)

Music

Music Tech Teacher (Elementary)

Memory Work

Chants, Jingles, and Songs
Association of Classical & Christian Schools List

Memory Work Resources
Circle Time and Memory Work

Reading Lists

Need ideas for what to read this year?  Try these booklists!
Mensa Excellence in Reading Award Program (K-12)
Houston Area Independent Schools Library Network Recommended Reading Lists (PreK-12)
Generation Joshua Recommended Reading (High School)
Robinson Curriculum Online Books (1-12)
Read.gov Booklists (Kids-Teens)
ALA 2013 Notable Children's Books (K-8)
Caldecott Medal & Honor Books, 1938-Present (Picture Books)
Geisel Award Winners & Honor Books, 2006-Present (Beginning Readers)
Newbery Medal and Honor Books, 1922-Present (Children's Literature)
ALA Great Early Elementary Reads (Early Elementary)
Hi-Lo Books for Upper Elementary (Reluctant Readers 3-6)
Flashlight Worthy Books  (user-contributed book recommendations, all ages)
About.com Children's Books  (many book lists, all levels)
California Reading List (K-12)
Accelerated Reader Book Finder (K-12)
A Classical Christian School's Reading List (1-3) **Added 7/21/13
A Classical Christian School's Reading List (4-5) **Added 7/21/13
A Classical Christian School's Reading List (6-8) **Added 7/21/13

Curriculum providers who make their reading lists available online:
Sonlight (PreK-12)
Veritas Press (K-12)
Tapestry of Grace Bookshelf Central (K-12)
Living Books Curriculum (K-High School)
Great Books Academy (K-12)
Robinson Curriculum Book List Sorted by Reading Level (1-12)

Science Booklists
Outstanding Science Trade Books for Students K-12
Science Trade Books by Topic
Readable Science

Math Booklists
The Mathematics Bookshelf
Children's Trade Books in Math

Social Studies Booklists
Notable Tradebooks for Young People

Science Experiments and Demonstrations

Experiments and Demonstrations
Hunkin's Experiments
Bill Nye Home Demos **Added 7/15/13

Physics
Physics Lab Manual
Supplemental Lab Activities

Science Fair
Science Fair Project Handbooks K-8 (includes instructions, sample timetable, resources, and grading rubrics)

Logic and Critical Thinking

Critical Voter Curriculum Upper Middle-High School+
Microsoft Critical Thinking Curriculum Middle-High School
A Formal Logic Primer Upper Middle-High School+

Recommended Resources

These books are not free, but well worth the cost, especially if they can be found at your local library.  Each of them contains helpful information that can be used to create your own free or low-cost homeschooling plan.

The Well-Trained Mind by Susan Wise Bauer
Home Learning Year by Year by Rebecca Rupp
Easy Homeschooling Techniques by Lorraine Curry
Easy Homeschooling Curriculum by Lorraine Curry
Easy Homeschooling Companion by Lorraine Curry
A Charlotte Mason Education by Catherine Levison
More Charlotte Mason Education by Catherine Levison
A Literary Education by Catherine Levison
The Three R's by Ruth Beechick
What Your Kindergartner Needs to Know  by E.D. Hirsch, Jr.
What Your First Grader Needs to Know by E.D. Hirsch, Jr.
What Your Second Grader Needs to Know by E.D. Hirsch, Jr.
What Your Third Grader Needs to Know by E.D. Hirsch, Jr.
What Your Fourth Grader Needs to Know by E.D. Hirsch, Jr.
What Your Fifth Grader Needs to Know by E.D. Hirsch, Jr.
What Your Sixth Grader Needs to Know by E.D. Hirsch, Jr.
Books to Build On:  A Grade-by-Grade Resource Guide for Parents and Teachers by E.D. Hirsch, Jr.
The Read-Aloud Handbook by Jim Trelease
Honey for a Child's Heart by Gladys Hunt
Books Children Love:  A Guide to the Best Children's Literature by Elizabeth Laraway Wilson

Latin and Greek

Latin and Greek
Textkit (Greek and Latin grammars, readers, and answer keys)

Latin
National Archives Beginner's Latin (not classical Latin)
Study Guide to Wheelock's Latin
Latinum
Wikibooks Latin
University of Texas at Austin Linguistics Research Center Latin Online
Latin for Mountain Men
Allen and Greenough's New Latin Grammar for Schools and Colleges
Fr. Gary Coulter Latin (Catholic)
Latin Handouts
The Latin Library

Fun Stuff
Headventureland (website of Classical Academic Press, has some Greek and Latin videos and games)
Greek Word of the Month
Latin Word of the Month