Dropbox was born on a bus trip. It's an idea for a remote storage system, available from any place through the Internet. Drew Houston often forgot his USB disks, so needed some workaround.
He thought that he could solve his own problem by himself. He wrote the code, but before he launched Dropbox, he made a short video. It was a presentation of Dropbox's capabilities, with Drew's voice commenting on what he was doing on-screen. He uploaded it along with a waitlist to Hacker News and Digg (Reddit's predecessor).
The video got 70k first signups the waitlist next day, mainly thanks to the inside jokes of the Digg's community that he included