I've been really digging the status pages for various web apps/services lately. As these services grow larger, and we grow more dependent on them, it's critical that we know how they're operating. They're typically one page sites with a single purpose - to report the status of the system. Usually, that is broken down into current status and recent status history, in which the primary focus should be the current status, and the history secondary. Often, they're completely separate from the rest of the site/service, so they can have their own identity as well. I love to see the results that come from those types of minimal requirements.
From what I've found so far, my favorite is the recently launched Github status page. It's a nice, clean design and the large, color coded header immediately conveys the current system status. The recent history is listed below, and makes it easy to scan for any recent issues. Also, it's hosted on Heroku, so it's external from their site, which is a necessary ingredient for it to be of use to anyone. Bonus points for auto-updating the status page via AJAX.
OpenDNS is pretty good as well. I like the big green button for "System is currently online". It would be much better though if they just swapped the Current Status and top section. My eye is drawn to the total activity chart which is not really that important. They also provide an IP address link for the status page since if their service is down, and you use them, you couldn't resolve the site's domain anyway.
Google's app engine status page is not bad, but like most Google creations, a bit heavy on data and light on design. At least you can easily see the current status. I like the use of friendly words like Now/Today/Yesterday so I don't have to think about what today's date is. I would prefer if the table went from most recent to least recent though.
I've posted some screenshots of other ones I've found in this flickr set. Let me know if you know of any other good ones, and I'll add them.