Adding Pre-populated Default Custom Fields

Hi I would like to automatically add a custom field with it’s value to every post and page by default. Is there a way to do this? I see some plugins and themes add pre-populated custom fields to your pages/posts, so there must be some way.

Not finding anything on Google (or the codex) that has really helped me out so far so I thought I would ask here. Perhaps I’m missing something in the codex.

Thanks. ??