What code search tools do you use?

One feature I really like about GitHub, which is seldom talked about nearly as much as many of their other features, is the search. Often if I'm trying to find some "real code" as an example to help me with my own work, searching for function definitions on GitHub's search has saved me so much time. Particularly when working with lesser known / used libraries.

Are there any other tools out there that provide a way to search quickly, in the same way GitHub's work, in which their emphasis is on search?

I'd add, that by search I mean cross-repo search. Not just on a single codebase.

EDIT: changed some words