Find and Grep are perhaps the most used command line tools for the Linux user or administrator. Terse but powerful, these two commands will allow you to search through files and their contents by almost any imaginable attribute or filter criteria: file name, date modified, occurrence of the some specific word in a file, etc. Combined with a couple of other standard unix utilities, you can automate and process modifications over a number of files that match your search.
Here are two blog posts by Eric Wendelin which nicely illustrate the basics of these two commands:
There are a number of other great unix utilities for file search, but knowing how to use find and grep is fundamental, as these two utilities can be found on the most basic build of every unix-like machine you come across.
Got a favorite command line hack that uses find or grep? Drop it on us in the comments.