[development] implementing "plugins"

Hi, I want to introduce a concept of "plugins" to my module. In this case the plugin merely needs to supply a single function that returns data. Is the best way to do this to have the plugin call a register function with the name of a function (and then call "eval"?). This is probably more of a PHP question than a Drupal question. Oh yes and is there documentation somewhere on what code Drupal automatically "includes"? Thanks, Dan