I was teaching recently and blew a few kids’ minds by doing this. If you’re using a mouse with a scroll wheel, you can zoom in and out on the entire Finder by simply holding the control key (by default) and scrolling up and down. You can change the key in System Preferences if you like:
I find it comes in quite handy when I’m coding out designs for everyone’s favorite browser, Internet Explorer (boo, hiss), and I need to take a closer look at things that misbehave. I was surprised to find out that this wasn’t necessarily common knowledge, and figured I’d pass it along. Happy zooming!