Among the #emacslisp programs executable form the user interface ("commands" in emacs terminology) which I mentioned in 「Emacs Lis で作る」 (2003), the command "file-multi-occur" survived until today (originally I called it "multi-occur" but a command with the same name appeared in the standard set of emacs. So I was forced to rename it).
There are also M-x locate and its DIY variant of mine M-x my-spotlight, but M-x file-multi-occur is the most often used emacs command for search in my file system.