OpenGrok - OpenGrok is a fast and usable source code search and cross reference engine.

searchcode - A source code search engine

GrepCode - GrepCode.com is a code search engine built by developers for developers to search and browse open...

Codase - Codase is the leading source code search company with advanced source code understanding and xml...

Krugle - Krugle is the complete enterprise solution for search targeted to the development organization. 

grep.app - grep.app searches code from over a half million public repositories on GitHub.