Software
Most anime fans use their PCs to watch Fansubs / DVDs, and often there are problems with playback, or just plain hassle.
Full Reviews
Windows
Quick-and-Dirty list
OS Independant
Windows
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K-Lite Codec Pack Solves all your codec related problems, as well as installing usful software such as Media Player Classic (see above) and
GSpot, which you can use to get information on those unplayable files. This is the
only codec pack that
RobM recommends, as it doesn’t give you unneccesary codecs, and is careful to uninstall older version before installing newer ones - thereby avoiding the headaches of conflicting codecs.
VLC While VLC is not as good in many respects as Media Player Classic (especially subtitle placement) it will play any region of DVD, making it very useful for watching many of the items in the library. VLC has all its codecs inbuilt so if you are using it you will not need the above packs.
Linux
Mplayer for playing general media files. It’s massively configurable, and plays literally just about everything!
XINE for DVDs. it supports DVD menus to a much better level than mplayer.
Something to be aware of: It is rumoured that the ability to play DVDs in Linux has got one guy in some big trouble with the DeCSS case.