Rak is a tool for searching directories for files matching a regexp, like a more convenient grep.
It gives you pretty highlighted output, and uses the Ruby regexp syntax.
To install type
gem install rak.
Rak is implemented in pure Ruby, so it should work on all platforms. Windows folk will probably find they need to use the –nocolour option (I will fix this next time I boot into Windows for any reason).
NB. Rak is an almost perfect clone of the Perl tool Ack by Andy Lester. He should get all the credit for the idea.