I don't personally have experience with online schools, but Acellus, Time4Learning, and Monarch are the online programs I hear people mention a lot. They are all pretty reasonably priced.
BJU and Abeka also offer video-based courses. They both cost more but are known for being more academically rigorous than the first three I mentioned.
When choosing homeschool curriculum, I like to read reviews on Cathy Duffy's website. Her reviews are thorough and can help narrow down the choices. A lot of her reviews are of traditional homeschool curriculum, but she does also review many online courses and programs.
Also, my website, Freedom Homeschooling lists free online curriculum for every subject, as well as all-in-one programs. However, all of the options we include are homeschool curriculum, not online schools.