I found a link for Courtney that gives tests over books then awards point that can be traded in on small prizes. I hesitated on whether or not post the link because I'm not overly thrilled about awarding kids for (or bribing kids into) reading. If you give kids good books with real story lines reading becomes the reward.
A child of mine specifically asked for AR tests. Knowing this child well and seeing that her head is constantly buried in a book regardless of points given I decided to use it as another tool at my disposal to test comprehension.
There are a few prizes you can have mailed after your child collects a certain number of points which could have some value with a really reluctant reader :)