Of all the terminal commands, the one I find myself using the most (several times a day) as a developer is this:
$ grep -lr “<search text>” .
It performs a recursive search (-r) from the current location (.) looking for the search text provided and returning a listing of matching files (-l).
Basically it does a “find in files” from the command line. Very useful for quickly finding things, and super-fast (no need to wait for a GUI to load and sluggishly do the search).