Slack is a cloud-based instant messaging service that allows people to virtually share content, files, or even make fun of their coworkers. It was the brainchild of Stewart Butterfield, who quit his job to create a video game company in 2002. He decided to focus on a photo-sharing website instead of an online game. The company was able to raise $17.5 million dollars in just two years and then use the money to build a new game. Slack is now the world's most popular social networking service.