The Google Webmaster Central Blog has a post about new widgets that Google has created that allow you to "embed a widget in your 404 page that helps your visitors find what they're looking for by providing suggestions based on the incorrect URL."
At Search Engine Land, Barry illustrates how this works:
For example, I set up a page at the Search Engine Roundtable at seroundtable.com/advertize.html, notice I spelled advertise, with a "Z." Google automatically notices that I have a page at advertise.html, and offers that as an option for people to visit. Here is a screen capture:
However, if you're using a Windows computer, it seems, that advertize.html page doesn't work at all and brings up a blank page instead if you're using Internet Explorer or Firefox. (It does, however, work in Safari for Windows and apparently Opera too.)
I'm not sure if that's a coding issue or what, but I'm assuming that wasn't intentional. (Two different users have already reported this issue, and I've been able to reproduce it. Thanks, Tom!)
Update: Tony explains the issue is due to the page not actually returning a 404 response. So it is my error and this should work fine.
Update #2: Google has fixed the issue and the script should work even on pages that return a 200 response.