sorry as per my knowledge tool is not availabe for yahoo advertising but you can use spyfu.com for keyword estimate.
Hi, I always do prefer Google adwords keyword tool because its best online tool to research best keywords for your website.
same here. you can choose if you want keyword to be exact or broad. although it is design more on ppc.
Before, Yahoo has the Overture keyword Suggestion Tool but it has been discontinued and no longer working. You may just use the Google adwords instead.
Yes a tool like that would be truly be very useful. Also I would like to have a tool like that for Bing also.