Before becoming a teacher I worked for a large tech firm so I am always interested in observing the effectiveness of educational software. I have found that the most important factor in that effectiveness is focus. Students need a goal and reenforcement that they are achieving that goal.
A program I recently used for my early readers illustrates what I am talking about. The site is SightWordsWithSamson.com. It is a program for early readers. It is effective because after completing each step of each level students receive a certificate that they can print out. They can also view a scoreboard that shows them green boxes for all steps passed, red boxes for steps failed and grey boxes for steps not tried. This not only allows me to measure their progress, it provides them with a concrete goal (i.e. turn all the boxes greem).