Which app did Marco Arment leave Tumblr to work on in September 2010?

In September 2010, Marco Arment left Tumblr to work on Instapaper.

Arment was Tumblr’s lead developer and chief technology officer during the company’s formative years. He joined David Karp in developing Tumblr, which launched in 2007, and helped build the technical foundation of the microblogging service. At the same time, Arment had created Instapaper as a side project for saving web pages to read later.

By 2010, Instapaper had become too significant to remain merely an occasional project. Arment left his full-time role at Tumblr in September and devoted most of his time to Instapaper, turning the reading-and-bookmarking tool into his primary business. He continued to have an advisory relationship with Tumblr rather than simply disappearing from its orbit.

The alternatives are easy to mix up because Readability, Pocket, and Flipboard also belong to the broader history of digital reading and content discovery. Instapaper, however, was Arment’s own service: it stripped away web-page clutter and saved articles for later reading, including offline reading on mobile devices.