Preprocess node in module
In Drupal7, both modules and themes are able to use preprocess functions in the same way, and a given theme hook can have many preprocess implementations, originating from both modules and themes. 

As with anything like this in the Drupal framework, knowing the order in which these functions run is important. Preprocess implementations from modules run first, and implementations by themes run last.

A quick search of Drupal core modules will give you examples in the node and rdf modules.