Now I am not a command line junkie, despite what you might have heard about my l33t t3ch ski11z, but grep is one damn useful tool, that really doesn't seem to have a good GUI alternative.
I have been working on an ecommerce site with lots and lots of template files and just finding the file with the code in it you want to change is a nightmare.
But using grep, I can simply:
grep -irl 'search term' cart/*.tpl
and it will return a list of all the files with that search term. It doesn't require me to associate files with it to make them searchable, it just works, and has saved me so much time. Its also damn fast, particularly compared to spotlight which feels almost glacial on my G4 powerbook.