SageThumbs - SageThumbs is a powerful shell extension allowing to preview enormous amount of image formats...
MysticThumbs - MysticThumbs generates thumbnails of many image types not natively supported by Windows.
Icaros - Icaros is a collection of lightweight, high quality, Windows Shell Extensions
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Pictus - Pictus is a fast, free image viewer that enables Windows to see thumbnails of supported formats, such as PSD. Give it a spin today!
Media Preview - Fed up with Windows Explorer not always showing thumbnails for all of your media files?