What Google logo can i use on my site

I used the G+ icon from https://developers.google.com/+/branding-guidelines. Hope this helps!

Google provides sign-in with g+ buttons and guidelines here — these are pretty specific and has links to actual image files that are authorized.

https://developers.google.com/+/branding-guidelines

After searching for quite some time, I come to the concluion that one should be able to use the Google logo at least as shown here, on the Google web site: https://developers.google.com/google-apps/marketplace/integrate There is obviously multiple UIs for this "sign in with Google" button that are accepted.

There are additional guidelines for using the Google brand here: developers.google.com/google-apps/marketplace/branding

This applies for Google Apps developers: not sure about other situations. Information on this topic is really scarce. The issue is mentioned here for instance: https://groups.google.com/forum/?fromgroups=#!topic/google-federated-login-api/oG5gk10W-R0 , but still, not precise answers.