Drupal is community based. Each little module, theme, issue, patch, debate, standard, approach, every single thing that connects to / plugs in / derives from Drupal is done by someone who's a member of the Drupal Community.
Even if you simply helped someone on IRC (freenode's #drupal channel) regarding Drupal, you can consider yourself a member of the community, a contributor (in a way or another).
Usually companies that work with Drupal, are the biggest contributors out there: they sponsor different events, they contribute fully polished modules, themes, features, they give sessions and trainings or offer office-space for those, etc.
Here, at Propeople Office, each Thursday, we organize free ad open Drupal Sessions. A Drupal Session is a small training-like event, where Drupal developers and themers and project managers that work with Drupal based projects, share their knowledge. Instead of showing one specific project and telling a story behind it, we try and focus on some techniques, modules and general aspects of Drupal: commerce, services, migration, search, etc
As I mentioned earlier, it's free and open: anyone can come to these sessions (but you have to let us know that you're coming ;) ). More than that, there's beer / soda and snacks after the sessions, and tons of geeky talks.
It's fun to share your experience. All-in-all we use Community Contributed things, it simply feels good when you give something back to the community and help others grow.