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Zoom player
Zoom player plays audio and video files,it can play any type of file available as long you download and install the right filters,it is highly customisable ,you can change aspect ratio,zooming, colour,brightness,gamma,sync. these adjustments can be made on the fly,because of this features an expert user will get more out of the program that just your average joe.it is very popular among home teather owners.Zoom player works on Windows 2000/XP/2003/Vista/XP64/Vista64
4-
VLC
VLC Media Player can too play audio and video files,it is open source,it main features are the ability to play uncompleted files that still downloading,access .iso files so that you can play files on a disk image,extensive filters,play High-definition recordings of D-VHS tapes,use a firewire connection to stream content to a monitor or HDTV,It can display the playing video as the desktop wallpaper,on windows ity can use third pary dll`s, plus it supports most operating sytems.
3-
BSPlayer 1.37.826 (Last free version)

BSPlayer can play most media files as long they are installed before hand,its multilingual,it has bookmark support,Remote control support,Playlist support,AVI files with more than 2 audio streams supported (multilanguage) and many more features, it is my personal favorite but..this is the last true free version,the newer free versions require the installation of an adware that must be installed for BSPlayer to work and therefore versions 1.38 and above suck.BSPlayer works on all windows versions.
2-
SMPlayer
SMPlayer is a Front-end for open source MPlayer,it has all the basic features like playing videos,DVDs and a special feature that saves the settings of all the files you play.it has multilanguage support,mouse wheel seeking,audio and subtitle delay adjustment,multiple speed playback,Etc. SMPlayer works on windows and linux.
1-
GOM Player
GOM Player is probably the best free media player available for windows,it can play the most popular formats as it is and if it can`t it will look for the codec online and allow you download it and install it,it can play AVI files that are being downloaded or are broken,in addition to most of the features found in other video players,GOM player has simple interface and uncomplicated controls that can be used by anyone,an awesome player. GOM player works on windows only.