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)

Memory Work

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

Memory Work Resources
Circle Time and Memory Work

Field Trips

National Parks Service Curriculum Materials
Teaching with Museum Collections Lesson Plans
Ronald Reagan Museum Curriculum Guides

Art

Various Media and Techniques
KinderArt
Teach Art at Home Free Lessons
See the Light Free Lessons
Making Art Fun
Art Projects for Kids
Art Smarts 4 Kids
Learn to Draw and Paint
WeeFolkArt (ages 4-6)

Drawing
Drawspace
Donna Young Drawing Lessons

Painting
Bob Davies Watercolor Tutorials

Artist Study
The World's Greatest Artists Volume 1