RAITA's omake page The New Year of 2007 was bound to be business as usual for the /a/ board of 4chan until on 4th of January, Anonymous posted an omake page from the doujin Schuppen Harnische by RAITA of doujin circle Zettai Shoujo, detailing character art for a dating sim he dubbed “Katawa Shoujo.” For reasons we will never know, someone's genius idea of actually realizing RAITA's idea of the game caught on like wildfire and soon suggestions ranging from tender love stories to depraved sex fantasies were running amok in the thread. The insane idea of creating an actual original game, based on nothing but a single picture and the sparse ideas RAITA had written in the margins titillated the imaginations of Anonymous so much that people became truly serious about the fledgling project.
Soon, there were development forums and for the next months, ideas and suggestions were flying around, with people trying to flesh out their ideas, drawing sample art, writing sample text, trying to control the chaos of dozens of people arguing and bickering about the direction of the game. From there, the most active people began the actual game development as a working group named Four Leaf Studios and the project truly took off in the summer '07.
The final chapter comes in the year 2012. For five long years Four Leaf Studios toiled on the project, with the eventual release sometimes seeming like it would never come. By the end of 2011, though, the end was in sight and the final push to complete the game began.
On January the 4th, 2012, Katawa Shoujo was released.
Source: The Developer Website.