iPad Air Smart Case has a built in magnetic wake and sleep sensor. Close the case and the iPad automatically goes to sleep. Open it, and it wakes up.
iPad Waterproof Case for iPad 4 - 3 - 2 with Screen Protector - Waterproof Case for Apple iPad 4 Retina Display - iPad 3 - iPad 2 - iPad - Tablet PC - Blue
JisonCase Handmade Leather iPad Air Smart Case - Cover and Stand with Full Body Protection and Magnetic Auto Wake + Sleep Function for the Apple iPad Air
iPad Mini Smart Case - Cover and Stand - Full Leather Protection - Cover and Stand with Full Body Protection and Magnetic Auto Wake and Sleep Function.