Blogger now allows commenters to use their OpenIDs to leave comments. Blogger explains it better than I can, but from what I understand, it will now accept your existing OpenID's from WordPress, LiveJournal, or AOL to name a few. Bloggers, note that you may need to change your blog's settings to accept them.
Commenters can continue to use a Google ID to comment or read more about OpenID and set up your own OpenID. The advantage of using an Open ID is that it can be used on other blogging platforms and websites, and it can be linked back to your blog or website.
The goal of OpenId is to be able to have one secure ID and one password that works at millions of websites without having to enter your email, birth date, and all that other information asked for when registering at a new site. Wouldn't that be great? I hope it catches on.