There are a few.
List of company or individually driven projects
Though it might be a simple product, the SNACK drawer has been available for sale for over 2 years. Probably the first 3rd-party community product for sale.
You also have the UART Expansion Card, RS-232 Expansion Card, other items like the Input Cover controller board. But you could argue that this LTE Modem Card is the first of its level. Something requested by many, useful to many, and more challenging than some others.