Software: KeyCastr
KeyCastr is a Mac OS X application that displays your keystrokes in a small floating window. It is intended to be used for screencasts or overhead projection, to emphasize what keystrokes need to be pressed when performing an action within an application.
As of version 0.8.0, KeyCastr's source is available on GitHub under the BSD license. Patches and comments are welcome.
Documentation (including developer documentation regarding how to write a custom visualizer) is still under development. Screenshots and example screencasts will be provided soon (the existing example was from the old 0.7.x codebase).
If you enjoy KeyCastr, please consider making a donation via PayPal. (Which probably means I need to put a PayPal link here, don't I.)