Adobe Reader - Adobe's Acrobat reader, popular web format for published media via the .pdf extension.
Flash Player - Macromedia's Flash player, a widely used format, mainly dealing with .swf files.
PowerPoint 2003 Viewer - If you aren't using Microsft Office/Powerpoint then this will let you view Powerpoint's .pps files.
QuickTime - Widely used video compression format, mainly .mov files.
Shockwave Player - Macromedia's Showave plug-in, handles mainly .dcr files.