There are literally hundreds of different file formats on the Internet. Think of them as languages (English, German, Spanish, etc.) Some of them your computer can read out of the box, others it needs a translator for. I have attempted to make a list of the most common media files and their viewers, plug-ins and applications:

Compressed archives: (though newer versions of Windows can handle basic compressed folder operations the interface is cumbersome, and you will need this to open many compressed files through email)

Winzip (.zip)

Movies and Sound Files:

Microsoft Media Player (.mpeg, .avi, .wav, .asx and many more) - (Even if you have a copy, check this site to make sure it says you have the latest version so you can play everything)

Real One (.ram .rm .ra) (previously RealPlayer, if you have the old version, you should upgrade for maximum compatibility)
(Real is especially tricky about the free downloads, be careful to not select any trials, bonus contents, etc. You just want the Free FREE version. Not the Free Trial version.)

Apple QuickTime (.mov)

When installing these players, I suggest not letting them make themselves the default player for all your media files. I use each one for what they were meant to do. For example, QuickTime is not so good at MPEG files, etc.

Web animations:

Macromedia Flash & Shockwave Players (you need both! from this page) - (Even if you have a copy, check this site to make sure it says you have the latest version so you can play everything)

Application Viewers - Microsoft Office and Adobe - use these if you don't have access to the application or one that opens that file format. They will not let you edit the file, just view it.

Microsoft Powerpoint Viewer (.pps, .ppt)

Microsoft Word Viewer (.doc)

Adobe Acrobat Viewer (.pdf)

General graphics files - Windows comes with some picture viewing software, but it is not very powerful. The following is a much more powerful software that supports dozens of file formats.

IrfanView

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