template-wide custom field

Looking for a way to include custom fields that are template/site wide, not just specific for individual pages or posts. Does not need to be entirely user friendly.

For example I would like to add custom data fields that would show up anywhere in the theme (header, footer, sidebar, etc) that could be added from the administration.

If anyone can point me to a resource that would be terrific. The only things I have found thus far related to custom fields for individual posts/pages.

Thanks!