Nullprpl was added to Pidgin on 5/30/2007 and debuted in version 2.0.2. See the Pidgin ticket and the original SourceForge tracker for more. Nullprpl was originally distributed as a stand-alone protocol plugin; the last release was mockprpl 0.3.
Beyond basic buddy list and IM functionality, nullprpl supports presence and away/available messages, offline messages, user info, typing notification, privacy allow/block lists, chat rooms, whispering, room lists, and protocol icons and emblems. Notable missing features are file transfer and account registration and authentication.
Nullprpl is intended as an example of how to write a libpurple protocol plugin. It doesn’t contain networking code or an event loop, but it does demonstrate how to use the libpurple API to do pretty much everything a prpl might need to do.
Nullprpl is also a useful tool for hacking on Pidgin, Finch, and other libpurple clients. It’s a full-featured protocol plugin, but doesn’t depend on an external server, so it’s a quick and easy way to exercise test new code. It also allows you to work while you’re disconnected.