Michael Nielson:"Ward Cunningham invented the first wiki because he was tired of
responding to user’s requests to update a website he ran. To save
himself time, he made the page editable, and told them to update it
themselves. He was shocked when this small change utterly transformed
the dynamics of the site.
One of the first widely used pieces of blogging software, Blogger,
was originally a small part of a much more ambitious project management
system. The project management system never caught on, but Blogger took
off.
The team that developed Flickr wasn’t originally building a photo
sharing service. They were building a multiplayer online game, and
decided to let players share photos with one another. When they
realized the players were more interested in sharing photos than
playing the game, they dumped the game, and built Flickr."