Accidental tech billionaires like Craig Newmark?

Are there others like Craig Newmark, who started Craigslist as a free newsletter and seems to have sort of happened into becoming a billionaire.

Craigslist does not seem like it was too focused on scaling or innovating - it only ever had one funding round - and that was after Newmark left as CEO. It really seems like Newmark happened into a viral success and tons of money.

Are there others like this? If not billionaires then big successes