there's a fascinating audio book called 1434 putting together a pretty convincing argument that many of the European "discoveries" where predated by Chinese.
I read the printed version of the book and found the nature and quality of the argument to fall somewhere between von Daniken ("we don't know exactly how the ancients built the pyramids, so it must have been aliens") and Thor Heyerdahl ("I can sail this thing so Polynesians must have come from Peru").
Also, noone really cares if some lone Chinese sailor "discovered" America, or some other land, if the knowledge about this discovery was lost. The reason Columbus is considered the one to discover America is because him landing on America's shores started a new era in history, not because he was first human there (there were Native Americans there, after all).
I read the printed version of the book and found the nature and quality of the argument to fall somewhere between von Daniken ("we don't know exactly how the ancients built the pyramids, so it must have been aliens") and Thor Heyerdahl ("I can sail this thing so Polynesians must have come from Peru").