Quia

Quia is a educational game site that makes creating games easy. I used it a few years ago. At that time I had 15 computers in my classroom. As students finished working on their classwork, they had the option of going to Quia and playing a game I created. Creating the games was easy, I picked the type of game I wanted to make and then entered the questions and answers that I wanted the students to see. Sometimes I out in the upcoming test questions, sometime I just gave them practice at what we were working on. The kids loved to play the game and didn't realize that they were learning stuff! They had a battleship game that your "hit" would only count if you could answer the question correctly.
By getting a Quia account, you can create games, track student progress, and see how many people are playing your game. But you don't have to have an account to use it. You can simply search for a game that somebody else has already created that matches what you want to be taught. If you have an account, you can copy that game and modify it, but if you don't, you can just have your students play it as it.
There is a 30-day trial if you want to try it out. I have let my account lapse in the past few years. I have changed schools, and with less computers I found my students just weren't using it enough to make it worth the expense. (I was paying for it out of personal funds, not my school's)
If you are looking for an easy way to make a game that will be fun for your students check out Quia!
http://www.quia.com/web