You’re scrolling down a list of files, trying to remember their contents. You did give most files a name that reflects the contents of the file, but a few leave you stumped.
Rather than open the file to find out what’s in it you can just select the file, then press the space bar.
OS X will present a quick preview of the first page of the file.

To close the preview, type command-W. You can browse the contents of most files by scrolling through them.
If you leave the quick view up and ‘touch’ (single click) another file, it’s contents will be presented in the quick view window that is open.
Tap the bottom bar and it’ll zoom for full-screen viewing. Click on Esc to return to the Finder view.
(Updated to add blog post tags & final line of text.)
Other articles in Code for life:
Before installing Mac OS X Lion
WWW database servers on Mac OS X 10.6.x, part I: Installing MySQL (3 parts)
Web browsers: poser tips for Safari users

Thanks! A v useful tip that I’ve just shared with several colleagues.