I would say a used/refurbished Nintendo DS, PSP, Cell phone with a web browser which can connect via wireless router, very old used PC or laptop which someone is selling very cheaply or giving away since they don't have the space at home for more then 1 PC, and an inexpensive ebook reader or tablet made by a unknown brand like Coby, Viewsonic, ePad,etc are the cheapest electronics for going online.
I would say the free cell phone which comes with a binding contact. granted you have to pay for the contact but the phone it's self is free and thus the cheapest device to use to go online.
True, I guest those free laptops, Tablets and Xbox 360, or other free internet ready device which one of the DSL broadband company in my city gave away if you agree to sign up with them for a 3 year or longer contract. But, the cancellation fees are huge.
