OK! Surely the answer is that the Americas were discovered by the first humans to get there - almost certainly east Asians. But the popular answer is almost always Columbus.
This view has been under challenge, by some popular writers who argue that other groups made it before Columbus. A strong arguement can surely be made for northern (viking?) Europeans.
A new book argues for Chinese visits also prior to Columbus, It's just released and is entitled:
Web reference: http://www.scribd.com/doc/168587593/Who-Discovered-America-by-Gavin-Menzies-and-Ian-Hudson