"Ack is a tool like grep, optimized for programmers". I knew the existence of Ack since a few years but I never gave it try. Now that I did, I'm positively surprised: it's easy to use and fast.
I was always fighting in Emacs between grep (what is the syntax?), igrep-find (why did it suddenly stopped to prompt for directories?), tags-search (ooops some of my files are not tagged) and rgrep (oh it did exist?). I installed ack-and-a-half for Emacs and its great. Ack-and-half tries to find automatically the root directory of your project but you can set ack-and-a-half-prompt-for-directory to true with (setq ack-and-a-half-prompt-for-directory t) for it to ask which directory to search in.
The Ack version on the website provides support for Clojure files. For ClojureScript files, you need to add the following lines to your ~/.ackrc file:
Et voilà, enjoy!