There’s Spotlight, which can find text inside of files as well as names of files. It’s under the blue magnifying glass in the upper right hand corner of your screen.
Click the magnifying glass and then type in the text you want to find and Spotlight starts to work. Spotlight finds emails, names and addresses in your contacts, as well as that text anywhere else in a file on your Mac. Your results are organized by what they are: documents, emails, folders, pdf files, and more.
Spotlight works by indexing everything on your hard drive (and other hard drives that you plugin) in the background while your Mac is on, even if you’re doing other things. So even if you just finished a document a minute before, the text will be searchable with Spotlight.
You can see instructions on the features of Spotlight at Apple’s support Web site: Mac OS X – Spotlight