The list below is a combination of both religious and secular sources for homeschooling curriculum.

Sonlight
http://www.sonlight.com/

Robinson Self Teaching
http://www.robinsoncurriculum.com/

Oak Meadow
http://www.oakmeadow.com/

A Beka Home School
http://www.abeka.org/

K12
http://www.k12.com/

Saxon
http://saxonhomeschool.hmhco.com/en/saxonhomeschool.htm

Alpha Omega
http://www.aophomeschooling.com/

Calvert
http://www.calvertschool.org/home-school/

The Classics
http://www.classicshome.com/prod_classics.htm

Time 4 Learning
http://www.time4learning.com/

Moving Beyond The Page
http://www.movingbeyondthepage.com/

Christopherus Curriculum
http://www.christopherushomeschool.org/curriculum.htm

Abbington Hill School
http://www.school-my-way.com/Index.html

Enki Education
http://www.enkieducation.org/