In short, Google owns SearchMash a new search engine where Google can test user interface designs without influencing the public opinion by placing the Google logo on the site. Danny has a huge write up on Google's New SearchMash Test Site if you want more details. Why did Google do this? Well, like I said, the Google logo has a huge affect on searchers perception on what is relevant and what is not, plus it taints what may be considered to be a "cool" feature and what is not. How do I know this? The Searcher Behavior Research Update in Toronto told me and it inspired me to write this at SEW, Is Google Search More Relevant Or Is There A Brand Factor?.
SearchMash features are listed here.
There is a lot of mixed feedback at the search forums.