A search engine. From what I know, its the only engine using Go (aside from Google.) Blekko uses Perl, DDG uses python (I think), Bing uses unicorn blood, and all the smallr ones use Jva and/or C++.
It's mostly C++ for indexing and lookup, but "Search" is a big topic, and there are Go, Java, and Python parts in the many systems that make Google's web search work. Go is growing in popularity within Google.
