As you might be able to tell from the sidebar, Something Emporium is now OpenID enabled.
This was one of the more interesting web standards I've used on the site. The standard itself is very complicated, consisting of a mish-mash of further standards and protocols (Yadis, XRI, XRDS); you might be able to whip up your own implementation of Trackback or Pingback over a few hours, but you're going to need a third-party library for OpenID.
Which is a problem. Even for a pervasive language like PHP, your options are pretty limited. There's no PEAR implementation, for example, and the support classes for the implementation that's supposedly coming have fundamental problems: missing semi-colons, faulty refactoring and no support for SReg (one of the most useful extensions of OpenID).
JanRain have a good implementation (good enough to call it the reference implementation), and that's what I ended up using. But it has its own problems: their commitment to support for PHP4 (4.3 no less! why?!) prevents the library from using some of the nicer things in PHP5 (static!) that I've come to expect.
So far, you can register and login using your OpenID. And the site will pull in any information it needs from your persona. Next, I'm going to work on making it really easy to use an OpenID to post comments. And provide a way for existing users to set up and manage their associated OpenIDs.
These days, you might have an OpenID and not even know it; Wordpress, LiveJournal, AOL and Yahoo all provide OpenIDs, for instance. Plus, there are free OpenID providers galore, and you can even use your own website by just adding two tags (I use http://www.dubdot.com/ for instance).
Once you have your OpenID, go ahead any try it out on SE. Despite the trouble I had getting it working, I have to say it's a really cool technology.