Google has launched an unbranded site -- SearchMash.com -- to test experimental search features that could one day show up in google.com.
A Google spokeswoman said in an email that the site was launched recently to test possible features for the regular search engine. There's no guarantee, however, any of the features would be used. Source
I can imagine it is helpful to keep an eye on search results in this new, test search engine. If you see your ranking plummeting in SearchMash.com, you could do some research and potentially prevent SE ranking from plummeting on google.com before it happens.