History's most famous college dropout never exactly dropped out. When Bill Gates left Harvard in 1975 to build Microsoft with Paul Allen, he took a leave of absence rather than formally quitting — and simply never returned, taking advantage of Harvard's flexible leave policy to chase the personal-computer revolution without burning the bridge. Gates finally collected a Harvard degree in 2007 — an honorary one — joking in his commencement address that he could at last tell his father he had gone back and finished.