Ultimate Guide to Playing and Transcoding Downloaded Videos
Here are our picks for essential video playback software:
VLC Media Player

Windows Media Player might be sufficient for Aunt Mabel, but real power users hardly ever touch the app. If you're serious about watching videos, nothing touches the VLC (VideoLAN Client) media player. This lightweight and versatile app will play just about any file format there is, whether it's an audio or video file. But that's not all it does. VLC will reveal which codec(s) a media file is using, it comes with basic converting capabilities, and you can even use it to set videos as your desktop wallpaper.
Free, www.videolan.org/vlc/
Media Player Classic

While not as versatile as VLC, Media Player Classic meshes modern video playback features into a lean app that works well with a variety of file formats, so long as you have the proper codecs. It also plays nicely with FFDShow.
Free, http://mpc-hc.sourceforge.net
Flash Video Downloader Suite

For anyone who downloads a bunch of Flash videos, this is a must-have application. Flash Video Downloader works with a number of sites, and its interface couldn't be easier. Simply plug in the URL or add multiple URL to the queue and you're in business. It will even check the URL beforehand to make sure it's a compatible link. Then it's just a matter of deciding whether to save the video as a Flash file, or let the program convert it into a different format. Groovy!
Free, www.flashvideodownloader.org/fvd-suite/
QT Lite

We happen to like QuickTime, but not everyone is a fan. Users commonly complain that the software is bloated, and then there are those who just refuse to touch anything related to Apple. If that sounds like you, you're in luck. QT Lite will allow you to play QuickTime files without the hassle of installing Apple's QuickTime Player, and it also supports embedded content.
Free, www.codecguide.com/qt_lite.htm
Real Alternative

Similar in concept to QT Lite, Real Alternative provides all the benefits of the Real Player without the bloated software. It supports all varieties of RealMedia content, and also includes a plugin for IE, Opera, Netscape, and Mozilla.
Free, www.free-codecs.com/download/real_Alternative.htm
Next, using and tweaking FFDShow!