SearchMash as the name suggests is a mashup of a Search Engine and is created and maintained by Google itself !
Surprisingly you don’t see any Google logo in the index page or the search result page. Only the TOS page seems to confirm that the site is maintained by Google !
When it was launched few months back SearchMash was a mashup of Google Search and Google Image Search.
There seems to be a lot of additions done since then and now SearchMash displays results from Google Search, Image Search, Blog Search, Video Search (Displays YouTube videos too) and Wikipedia.
However the number of search results returned by SearchMash is different from the count returned by a normal Google Search. As the “Feature” page indicates, Google is running this site to test user interface ideas and is not a full fledged mashup search engine !
Use SearchMash and do give feedback on the features that you liked and disliked