Indexing is better, but ack is still nice to have for ad-hoc searches.

Ack plugins for Emacs:

http://www.emacswiki.org/emacs/Ack

http://rooijan.za.net/code/emacs-lisp/ack-el

https://github.com/jhelwig/ack-and-a-half

http://www.emacswiki.org/emacs/FullAck

Ack is great