OpenGrok
Fast and usable source code search and cross reference engine
OpenGrok is a fast and usable source code search and cross reference engine. It helps you search, cross-reference and navigate your source tree. It understands various program file formats and history from many Source Code Management systems. In other words it lets you grok (profoundly understand) source code and is developed in the open, hence the name OpenGrok. It is written in Java.