One of the simplest games for math education is the flash card. It's easy enough to make a set of them by simply writing a math problem on the front of an index card and the answer on the back. However, with a set like that, it's also possible for the student to simply learn the right answers without having to learn the math itself. Fortunately, that's what something like Flash-based flash cards are for. By randomly generating cards set to a specific difficulty and function, you can easily test your student on a variety of randomly generated mathematical questions.
Sometimes a little competition can interest even the least interested of students. This is precisely what Grand Prix attempts to do: by setting your math student against other players online, they have a direct challenge. By getting the multiplication problems right, the car gets a burst of speed, which will propel it toward the finish line. Answer the question faster than your competitors, and you'll win the race. To speed the process up, you can use the 1 through 4 keys on your keyboard to answer the questions. As it is an online multiplayer game, you will be playing against live people over the web, but there is no direct interaction.
One of the best ways to learn how shapes work together is with Tangrams. These Chinese puzzles take a number of pre-determined shapes, and it's up to the player to arrange them so they look like the target shape. The online version hasn't got the same tactile feel of the original game, but is still entertaining. The puzzles are challenging, but getting them right is rewarding. If one puzzle is particularly challenging, the solutions are also available.
Learning the ins and outs of a business is something almost synonymous with the concept of a lemonade stand. Unfortunately, setting up a lemonade stand isn't always easy--maybe it's the dead of winter, or your street just doesn't work for a stand. That's why virtual lemonade stands are useful tools for this same thing. Kids will need to balance the right amount of advertising, the right number of cups of lemonade, and the cost per glass for the customers in order to succeed for a full month.
This game is a little more challenging, as it requires not only math but logic as well. The game has a field full of numbers, and your goal is to use the red square to clear them all. The square picks up the number, and obeys a few rules that the first few levels of the maze explain to you as you play. This is probably not a game for the same audience as the others on this list, but it provides an interesting challenge for more adult math students.