In most cases, you can strengthen your feet with exercises which require no special equipment or according to some research, just by spending some time walking around barefooted or in minimalist, thin soled shoes. However, for the gadget oriented, there area a couple of devices available designed to help you strengthen your feet. This is not an endorsement but a listing of what is available as far as I know.
The Foot Gym is designed to stretch your achilles tendon as well as strengthen the small muscles in your foot that move your toes up and down.
Elgin Archexerciser, designed to help strengthen the arch muscles in the foot.
strongerfeet.com 