Suppose I own a patent that has not expired. How do I legally and reliably transfer it to the public domain?
I really know nothing about this. However after a bit of searching I did find the concept of "Patentleft" which seems to be an open source license for patents. Here is the Wikipedia page which is unfortunately brief. I also found the Open Invention Network which seems to be targeting patents related to Linux specifically. Lastly, this paper seems particularly relevant but is too long to summarize here.
It is possible to file paperwork with the patent office to disclaim all or some claims of a patent. I do not think there is a form for this. IBM famously disclaimed US6329919B1 System and method for providing reservations for restroom use. I would link to a copy of the disclaimer they filed but the application is old enough that the image file wrapper is not on line in PUBLIC PAIR.