Universal Viewer is a lightweight and versatile file viewer that can open virtually any file format including PDFs, Fonts, Flash Video, SWF animations, Microsoft Office formats, Images, Music and Video Files.
This will be a blessing for the not-so-techie crowd who have a hard time remembering the file associations.
Universal viewer can also be used a web browser or an HTML code editor. When you open an HTML webpage inside the viewer, it can either display the HTML file as a rendered webpage or as a source editor. That's so useful for making quick edits to your HTML files.
While you can open all common formats including Photoshop, AVI, FLV, etc - you can include support for additional file formats by downloading free file viewer plugins.
Unfortunately, this gem of a tool cannot be used as a Microsoft Project Viewer which is clearly the most sought after viewer on the web.
Universal Viewer (ATViewer) | Lister Plugins [H/T Abdul Aziz]