I supposed Christopher Columbus is usually given credit for realizing this, though the ancient Greeks appear to have had a clue:
http://www.monsoonco.com/blog/time-to-retire-the-flat-world-metaphor
It’s bumpy and curvy all over
This from the blog of Sandeep Sood, author (not illustrator, that’s been outsourced) of Doubtsourcing, a comic strip portraying outsourcing.