(File this tool under kinda cool but not likely to be useful.)

May I present to you the Yahoo vs. Google Web search tool.

Basically the tool looks at the results from Yahoo and Google, finds the similarities and then gives you a graphical display of what results are in both search engines.

Here is a screenshot:

The big takeaway from this for me is that there are some queries that are pretty close on the two engines and some that are totally different. This illustrates that optimizing your site for both engines is not a very easy thing to do, but not impossible.

Enjoy!