I admit, I mostly stole this post from here, but I feel it is valuable enough that I need to repost it.

Adding OpenID to your domain is seriously a piece of cake. For example, I own www.schuetzler.net (this site). By adding the following lines to the <head> of your homepage, you can use Google's OpenID, connected to your Google Account, to login to any website that allows OpenID. [Note: the second href= is your Google Profile URL. If you haven't set up a Google Profile, you can do so here.

<link rel="openid2.provider" href="https://www.google.com/accounts/o8/ud?source=profiles" />
<link rel="openid2.local_id" href="http://www.google.com/profiles/101917908304099087510" />

That's seriously all you need to do. Once that's set up, you can log into any site that accepts OpenID (including this one) by entering your domain for the OpenID.