The awesome guys from the Windows Vista Magazine revealed a great Windows Vista secret recently that I wanted to bring to a broader audience.
When you're browsing around the file system in Explorer, it's often useful to be able to quickly switch between different folder views. If you're looking at a folder with lots of long filenames, maybe the Details view works best. If you're looking at a folder with lots of photos, maybe a small thumbnail view works well; for a folder with lots of documents, maybe a larger thumbnail view works better. Now you can use the Views button on the toolbar to modify this - click the button to toggle between views or use the down arrow to get access to the slider - but it's quicker to use a shortcut. Simply hold down the Ctrl key and use your mouse scrollwheel to resize the icons from small to extra large.
What's really cool about this, as Jon points out, is that it also works on the desktop itself. If the desktop has focus, simply do Ctrl+mousewheel and you can go from the regular 48x48 icons to full 256x256 photographic-quality renditions. For those of you who have a desktop filled with documents and shortcuts, you can of course also use this feature to cram even more on your screen so that you never have to create a folder again!