Arcademic Skill Builder: online educational video games that offer a powerful approach to learning basic math, language arts, vocabulary, and thinking skills. Please note that is an American website so not the best option for spelling or geography.
Prodigy: free, Canadian based, grade appropriate math game that allows me to track student progress and give assignments. Parents can choose to get reports as well. Hints are given to students through the question mark. The more students play, the more prizes (clothing, pets and magic attacks) they earn. Most children love it! There is a paid version, but there's no need to buy.
Ten sites below courtesy of Teach for India.