"Is there a tool that will allow you to type in a domain name and see the top keyword hits on that domain?"
Not currently that I'm aware of... and I don't see much use for it. Who cares what search terms they ARE being found with... Don't you REALLY want to know what search terms YOU want to be found with?
Keyword research can be an expensive part of SEO. Here is a handy tool to see what terms are being used to search for stuff on the net on Overture.
http://inventory.overture.com/d/searchinventory/suggestion/ Type in a key phrase and see what ordinary people are using to search for things on the net. Then take that terms and go to google, type in the same search term in Quotes to see how much competition there is for that specific term.
Example: "real estate"
Overture says 2,268,491 people searched using this term last month.
Google says this:
Results 1 - 10 of about 28,200,000 English pages for "real estate"
So for those 2.2 million searches, there are almost 30 million pages out there competing for front page position. You may want to find some search phrases that have better odds then that and are more geo-targeted, and focus on those for your target search phrases for your website and forget about what where other sites are getting their traffic.