Windows 7: The preview pane in Windows Explorer for Windows 7 is much better than the version in Vista (with built-in support for more file types), and it has a handy keyboard shortcut: ALT+P.
You can access the preview pane via the icon in the top right of Windows Explorer, but for faster access, just hit ALT+P to show or hide the pane.
As the video above shows, the preview pane can be resized for a more detailed view. This is especially handy for images, since the previews are much larger than the thumbnail views. You can also play video (as well as audio) files from within the preview pane.
Some file types require the program to be installed (e.g., Microsoft Office or Acrobat Reader), and some files also just don't preview even if it is installed (e.g., OneNote). File types that will show up in the preview pane include HTML, plain text and rich text, XML, and the aforementioned images, videos, and music.
The next time you want a peek at an image or file that is supported, hit ALT+P.
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