It's an emerging (not really) concept worth keeping an eye for.
Currently the ones I'm aware of are Google's Code Search, Koders and Krugle. Of them Krugle seems to beat all others to smithereens.
I just found http://www.codase.com/index.html to add to the above ones. Claims to search over 250 million lines of code which makes it as desirable as the others (with an exception to google's which is unfortunately rather useless).
What I did like on this one was the search methods. Very interesting. Search for class names, with or without specific interfaces, function definitions with or without specific return types, member functions, etc...