Windows: Definitely NVDA (github.com/nvaccess/nvda)
Linux: AT-SPI, ATK, Orca for the graphical side of things. The console/terminal is pretty well supported.
Thanks for those links.
I recently looked for a plugin that reads highlighted text in firefox that "just works"... but found limited success (this was on linux). But really great to see that there are large-scale, OS-level already established FLOSS projects out there.