The t.m.k firmware collection. The files are available at this github page.
This collection can be used to power converters for both common (PS/2) and uncommon (Sony NEWS) keyboards. You can also use it to run teensy powered KB mods. Most recently alaricljs worked with hasu to provide support for the HID Liberator produced by bpiphany.
Use the documentation found on the git page, more info will be made available here as needed.