Google Kythe - An open source project from google to index code, based off of the older Grok system
CodeXCavator - Open source on sourceforge which performs code indexing. However, it seems outdated and unintuitive
Hound-Search - Another open source GitHub repository with code for source indexing
All of the above products are open source and copyleft. There are no established companies developed around the system. This indicates that the playing field for such a product is open. Additionally, there seem to be no related patents. However, different companies may have their own, closed source, internal versions of this software. This indicates that the product can be sold to smaller companies to save them development time.