Anybody who has used command-line systems for a serious amount of time will love grep. But today I stumbled across ack, which (for many things) is better than grep and a whole lot nicer to use.
The best bit? It’s pure Perl, therefore also uses real Perl regular expressions. Yes, there might be grep --perl-regexp, but nobody bothers compiling that in. Plus ack has some other neat features.
See more at the ack website.