My .02 (euro)cents..

I created a telnet client in perl, to which modules (plugins) can be added. The basic plugin has default values/methods for everything, each of the actual plugins inherits from this, and implements only the bits it actually needs. (Eg: The Alias module only implements the hooks to the user input methods, whereas the Trigger module needs the server input only). (Hmm, this'll probably only make sense to anyone who has played MUD or similar, whatever, you get the idea :)

C.