I always really liked the GitHub contribution graph as a dense data visualization and at some point joked with my friend that I'd wear it on a t-shirt. Somehow this turned into a mug, and the rest is history.
It was cool seeing people I really look up to buying these and sharing it, like Wes Bos.
These were all printed on demand so there was a big challenge of figuring out how to generate the graph images on the fly. For some strange reason I decided to build all of the ecommerce functionality out myself (ironic becuase I eventually ended up working at Shopify).
This was a really fun and rewarding project - I still have my own mug on my desk.