SearchMash is a low-profile site created by Google, that mixes Google Search with Google Image Search and adds some interesting features. The concept is similar to the old A9.
Search results are numbered and you can reorder them. Clicking on the green URL, you'll see a list of options like "cached copy", "similar sites". You don't have click on the search box to type your query. There is no search button, you just have to type enter. Google shows the number of visible search results in the top right corner. Clicking on the "more" link, Google loads a new set of results using Ajax and scrolls to the first result of the new set using a nice animation (this seems broken in Firefox). Actually the entire site uses a lot of JavaScript and Ajax, and it can't be used if you disable JavaScript.
For the moment, SearchMash is a playground for small ideas that could be the foundation of an entirely new Google site. Although this site is mainly for Google testers, it's interesting to see it growing.
{ Found by Pete Warden, who has an interesting site called... SearchMash. Via Blogoscoped Forum. }
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Restrict your search to your favorite sites, soon at Google