Today is the last day you will be able to use Yahoo Site Explorer.
Yahoo announced late on Friday that they are closing it down today, November 21st. They said:
With the completion of algorithmic transition to Bing, Yahoo! Search has merged Site Explorer into Bing Webmaster Tools. Webmasters should now be using the Bing Webmaster Tools to ensure that their websites continue to get high quality organic search traffic from Bing and Yahoo!. Site Explorer services will not be available from November 21, 2011.
This comes with no surprise. Yahoo notified everyone that it would happen this year but since today is the day, it is some what of a shocker that it has finally happened. It is basically the last nail in the coffin for Yahoo Search.
Update: As of Tuesday, November 22nd, it is now closed and here is what the closure page looks like: