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)