I love Spotlight on Mac but wanted to explore a few other tools. Here are a few of my favorites.
I really like DocFetcher. It's a Java program that runs on all Operating Systems and you can even put it on a USB with an index that leads to other files on the USB. DocFetcher can index all sorts of files including PDF, Microsoft Office, ID3 tags within an MP3. I found it easier to work with then beets.
mdfind is the Terminal equivalent of Spotlight. It's very powerful and most important of all, scriptable. Below are a few common commands.
ack is a search tool designed for code. It's built to be a customizable replacement for grep with higher speed, automatic recursion, and is able to skip unimportant files.
Grep is not my favorite replacement to Spotlight but it has its use cases. Warning: This may take a very long time.