How to play AMVs
So you've downloaded an AMV but are having trouble playing it? Well you've come to the right place, because I will show you how to quickly and easily play almost any type of video.
Playback on Windows
The best way to play AMVs on windows currently is by downloading the Combined Community Codec Pack.
This package contains an excellent player called Media Player Classic Home Cinema, as well as some extra decoders and things that you may need. I would generally tell people to stay away from anything called a "codec pack", because they can often cause more harm than good, and sometimes cause your videos to STOP working correctly! But in the case of this package, you can trust it, because they try to keep the number of installed components to an absolute minimum, and everything has been tested thoroughly. You can also receive support through their forums.
That is all you should need! However, if for some reason you don't want to use it, or it's unable to play some obscure video format that you have, you might also want to try downloading VLC Media Player. I don't recommend this player as highly, because it has been known to have problems with certain formats at times. I do think that this is probably your second best option though.
If you manage to get a hold of something that you can't play with either of those players, then it can be helpful to find out exactly what you have. A nifty little application called MediaInfo can tell you all sorts of useful information about video files. If you have a video and you don't know what it is, it can be helpful to check it out with this application before asking for help.
Playback on Mac
QuickTime is the default media player on Mac systems, but out of the box it kind of sucks at playing some filetypes. However, you can add support for many additional types of files by installing Perian.
If you would rather not use QuickTime, or if you are still having trouble getting a file to work, then I would recommend MPlayer OSX Extended.
Playback on Linux
For playback on Linux, you will definitely want to go with MPlayer.