A couple of years ago I reviewed a product called Mathletics. You can read the review here. My son enjoyed this program enough that we continued our subscription after the review period.
Each year this organization hosts the World Education Games. It is a free spelling, math, and science competition that your kids can join and compete with other countries. There are even prizes to be had! Unfortunately, it has already started, but there is still time to get in on World Science Day tomorrow! It is a perfect opportunity to set aside your regular science studies for the day and have a little healthy competition.