Where to Shop

One Stop Shopping for Curriculum
Homeschool Buyers Co-op - I always check here first if I know what I want. They have some great deals.
Rainbow Resource - One stop shop for everything you might need to homeschool. Their catalog is the size of a phone book. They do not have PDFs. Currclick is good for that.
Cathy Duffy Homeschool Reviews - Not sure what you want? She has thorough reviews with recommendations for each type of child and links to where to purchase.
Christian Book

Online PDF Download Sites
Scholastic - Teacher Express - watch for Dollar Day Sales
Teacher FileBox - If you like Evan-Moor material, have multiple grades and have a decent printer, it might be worth purchasing a subscription to Teacher FileBox. I usually search the Evan-Moor site, copy the ebook number and then search it in TFB for a quick search.

School in a Box

Ambleside Online
Bob Jones University Press
Bob Jones University Press Distance Learning
Calvert School
Five in a Row - Unit study using literature complete with list of days activities.
K12 - Individual Courses or whole package on a monthly subscription - Teacher assistance available for a higher price.
LifePac - Alpha Omega- Grades K-12, Bible-based, Worktext-based
Memoria Press - Classical curriculum
My Father's World -  Complete Curriculum for Homeschool and Christian School. Combining the best of Charlotte Mason, classical education, and unit studies with a Biblical worldview and international focus
Oak Meadow - Complete curriculum that involves the whole child, supports their natural development, adapts to learning styles, and creates a caring and supportive relationship. 
Peace Hill Press - K-2nd Grade, but adding more
Switched-on Schoolhouse - Alpha Omega - Grades 3-12, Bible-based, Computer-based,
Veritas Press - Print and online options