At Pattern Insight (http://patterninsight.com) we have build a source code search engine. We do not give this to our customers as a standalone tool most of the time but it's the underlying technology for our product, Code Assurance, which helps companies eliminate bugs from their releases.

We use it internally to search our code / libraries, if anyone is interested in indexing/searching his own code, especially if it's open source, I would be happy to provide a copy. Email: spiros at patterninsight.com