I found this awesome site this morning. ExploreLearning.com has over 300 "gizmos"--interactive, web-based games/simulations--that teach concepts in middle and high school math and science. It's particularly well-suited to visual and kinesthetic learners and works equally well for homeschool/after-schoolers/individual learners and class learning via computer/media lab.
ExploreLearning has already correlated their gizmos with major textbooks and state curriculum standards, so homeschoolers who are worried about keeping up with the public schools can be sure they're covering everything they need to. Just click on your state, the standards are copied word-for-word with the list of appropriate gizmos just beneath. The gizmos all come with five or so assessment questions at the end of the activity and reports the score to the student and to anyone designated as teacher, so you can keep track of how much your child is doing and comprehending easily.
Xavier and I played around with the Chicken Genetics gizmo and had lots of fun. You get five minutes to play without registering, which isn't quite enough time to complete an assessment. ExploreLearning does offer a free 30-day trial subscription, after that there is a subscription system available. I plan to use this to extend the boys learning come fall. Too many textbooks makes Jack a dull boy. :D