On my site i would like to sign in with a google account. I plan to use openid but i would like to see signing in with google has more benefits. I notice in the past a few sites has the ability to sign in with a google (gmail) account and IIRC they all did NOT support openID (i could be wrong)
How do i 'sign in with google' ?
You can look into openId (http://openid.net/) which is what SO uses, and is supported by Google.
If you plan to use OpenID, use that. Google is already an OpenID 2.0 provider.
Google's OpenID provider is located at: https://www.google.com/accounts/o8/ud
(NOTE: There's no point visiting that URI in your browser, but it does work for OpenID.)
This is primarily addressed on the Accounts API page, which also addresses OAuth and the hybrid and proprietary login systems. Depending on your site, you may also want to use Friend Connect, which is an OpenSocial container that internally uses OpenID for authentication.
I'm of course biased towards Friend Connect, since I'm the DPE for that project, but you're probably better served directly using the OpenID provider unless you're also doing stuff that involves a social graph.
You may be interested in RPX which is an all-in-one solution that lets people choose which identity provider they would like to use to log in to your site. Not only are Google and OpenID supported, but many others as well.
RPX takes care of all the details of interfacing with each identity provider, and gives you a common API to work with.