My favorite game is "Hang Mouse" which is a variation of the "Hangman" game we used to play when I was a kid. In this version you pick letters until you've figured out the spelling word. When you pick a correct letter, the mouse sneaks over and takes a piece of cheese. If you pick an incorrect letter, the cat wakes up a little more each time until he finally captures the mouse.
There are a number of other games on the site as well.
At the end of the week there is also a "test me" feature on the web site. It will actually say the spelling word out loud, and your child has to type in the correct spelling.
I am finding this to be an excellent way to add entertainment to our spelling lesson! The kids are learning their spelling and vocabulary words, but they are enjoying it and remembering more while they are doing it!