Windows only: You may have noticed that Windows only supports thumbnail icon previews for a handful of image formats. Free utility Thumbview adds in over a dozen more to make sure you can preview all your images.
It's annoying to look at a directory of image thumbnails and have some be proper thumbnails, while others are just icons of whatever program they are associated with. Thumbview adds support for 19 image formats, including PNG, PSD, TFF and more.
According to the documentation on the Thumbview site, it's light on system resources and only springs into action when rendering a new batch of thumbnails. On our test machine it was quick and seemed no slower or faster than Windows typically is when re-filling thumbnails. For an alternative application, check out previously mentioned Xentient Thumbnails. Thumbview is freeware, Windows only.