The notorious Keyword suggestion tool from Overture (Yahoo) seems to be down for the last few days. I have noticed it myself and I have noticed a few other posts mentioning that its probably gone forever. I don't think that its a permanent shut down of the tool otherwise there would be at least a Note on that page indicating it. On the other hand there is not even a note that the tool is out of order ... well we will wait and see.
Due to this, the search engine queries estimates tool I just released a couple of months ago is unable to function (its getting the results from he overture database). If you are looking for a keyword tool please refer this list of keyword suggestion tools.