Earning extra *fun* computer time is one of the incentives we have for the kids to finish their work on time, respectfully, completely, cheerfully, etc. In the past the kids have stuck mostly to PBS Kids and Starfall, but we've been branching out to try new things the last few weeks and I'm really happy with these two sites that we have recently been added to our safe list!
National Gallery of Art: NGA Kids Zone Here the kids get to interact with art by redesigning and animating famous paintings or creating their own. They can also print the images or (what we do) save them to PDF.
National Geographic Kids This site has a ton of animal research information but also some great games and activities. Caleb particularly liked the recycling sort game where he got to catch pieces of trash falling from the sky and then sort them as recycle, trash, or compost.
Let me know what you think if you try them and if you have any other favorites!