Autocomplete of craft()->craft_pluginservice->foo()
It's 95% an IDE issue.  Craft can take advantage of this because it provides PHP DocBlock property information in WebApp (which is what craft() is a reference to) and IDE's like PhpStorm are smart enough to analyze that and find the classes you're referencing for autocompletion.

It's impossible for Craft to let plugins dynamically add properties to that DocBlock, but I have seen some clever plugins provide their own abstraction around craft() so they can add their own plugin specific DocBlock info that enables autocompletion around services.