Which startup was founded by Stewart Butterfield?

Stewart Butterfield founded Slack.

Slack grew out of Tiny Speck, the company Butterfield created to develop the online game Glitch. While working on Glitch, the team built an internal messaging system to coordinate development. When Glitch closed, that communication tool became the basis for Slack, which Butterfield announced publicly in 2013 and released in 2014.

The name is often confused with other workplace-communication products, including Discord and Microsoft Teams. Discord was created primarily for communities and gamers, while Slack was designed around workplace channels, direct messages, searchable archives, and integrations. Butterfield is also associated with Flickr, the photo-sharing service he co-founded before Slack.

Slack became a major business-software company and went public through a direct listing in 2019. Salesforce later agreed to acquire Slack for $27.7 billion in 2020, making Butterfield’s startup one of the most prominent examples of a consumer-style messaging idea becoming enterprise software.