Ploopy is an open source trackball based on the venerable Microsoft Trackball Explorer—which is no longer in production but is popular enough demand a hefty price in the secondhand market. Ploopy is available to pre-order in kit form. I prefer trackpads these days, because the gestures for accessing Mission Control and App Exposé are too good to pass up, but before that I preferred trackballs to mice. I went from the Microsoft Trackball Explorer itself through every generation of the Kensington Expert Mouse—the most recent of which is still available new today.
I also use an Ergodox EZ keyboard, and part of the appeal is the open source hardware. I’m hoping to be able to use this keyboard design for the rest of my career. But I’m not sure the open source hardware model works as well for a pointing device. One of the reasons it works so well for keyboards is that the foundational technology has changed to little over the years. Cherry MX switches were developed in the 1980s and, as far as I can tell, they’re similar to the product that’s the industry standard today. Pointing devices on the other hand, gone through a couple of upheavals like the optical mouse and touchpads themselves.