It looks like your server is configured to serve pages for http://www.example.com as well as http://example.com
These are considered to be two separate domains. Besides showing up as 'referrals' in Analytics, your content (and its ranking) may be devalued because it is duplicated and can be accessed across multiple URLs.
but what about this? I have "google.com / referral" showing up in my analytics but don't have any links to google (except the analytics code). Any ideas why that is or which part of google is referring to my site? Organic results still show as 'organic'.
Referrals from google.com means that a browser sent this as the referring URL. Maybe a link to your site exists somewhere on Google. Not necessarily a link from search, could be any of their many other services. And maybe, more likely IMO, the referral string was spoofed by the browser.