Henderson is best known as the co-founder and
chief technology officer at
Slack, as well as co-owning and developing the online creative community
B3ta with Denise Wilton and
Rob Manuel; being the chief software architect for the photo-sharing application
Flickr (originally working for
Ludicorp and then
Yahoo); and writing the book
Building Scalable Web Sites for
O'Reilly Media. He has also worked for
EMAP as their technical director of special web projects and is responsible for writing City Creator among many other websites, services and desktop applications. Cal was the co-founder and VP of engineering at
Tiny Speck, the company whose internal tool transitioned into Slack. Henderson's connection to
Stewart Butterfield and Slack began through a game developed by Butterfield's first company, Ludicorp, called
Game Neverending. He ran a fan website dedicated to the game and broke into an internal Ludicorp mailing list. Instead of repercussions, Butterfield hired Henderson to work for his company. == Personal life ==