Here is a list of homeshool sites we've been using.
Khan Academy - We only use this site for Math. It also has Science, Economics & Finance, Arts & humanities, computing, and SAT test prep. My son watches the video lessons and then answers the math questions. The site will explain a problem again if you get it wrong. I can check his progress and see where he is doing well or struggling.
TeachingBooks.net - I use this site to find lesson plans to go with books we are reading.
Wordly Wise - We use this site for Language Arts vocabulary. There are also books that you can purchase to go along with it, but we just use the website. The lesson says the word, reads the definition aloud, and then uses the word in a sentence. My son enjoys playing the games to practice his vocab words.
Quizlet - This site is great for just about anything. It has flash cards for vocab for just about every subject. Right now we are using it for Science and Spanish. You use the flash cards to learn terms. There are review games to play, spelling, and even a test to check your knowledge.
Discovery Education - Right now I've just been using this site for lesson plans. They have plans for K-12 on all the core subjects. You can search by grade level or subject type. The site also has puzzle makers and virtual field trips.
Easy Peasy All-in-One Homeschool - We started using this site for some of our core subjects. It is a christian homeschool curriculum, but my son and I just skip the christian stuff because it's not our beliefs.