App available for kids

I just bought an iPad last week. The kids love my new gadget. They learn it first because they have it at school. I am getting use to it now. I am faster using my laptop than my iPad. My kids love to play game as they play it at school. Do you know any educational App for kids that I can download it for free? I want them to play at the same time learns something. Do you have an iPad too? Please share me some information on what Apps are great to download on my iPad.