Nautilus (the GNOME file manager) is able to display thumbnails for several file types: images, video, audio (holding you mouse over a sound file for a few second), text, pdf, etc.

David D. Lowe has developed a small script for OpenOffice, which not only creates a preview, but it also displays a logo of the application in the bottom right corner (for example, Writer - see screenshot at the end of the post). This initially worked only for OpenOffice.org Writer, but Guiodic modified it so that it can create previews for all of the file types supported by the OpenOffice.org suite.