If your system is dealing with media files such as video, audio and image you’d better get some samples to make sure your system can play most of them or at least can match (or beat) the competition using the same platform.
So I’ve collected some links for just doing that.
mplayer test samples: http://samples.mplayerhq.hu/ – Over 42 GB / a few thousand files of diverse audio and video files.
Microsoft HD Showcase – http://www.microsoft.com/windows/windowsmedia/musicandvideo/hdvideo/contentshowcase.aspx – A must to test wmv, wma and wma pro decoding capabilities.
For testing MKV videos, you’ll most likely need to download some videos using bittorrent or emule. To search for video you can use sites such as http://www.isohunt.to or http://www.verycd.com/. Very CD is actually one of the top website in China in terms of traffic. You can also convert some other videos to MKV using some MKV editors my favorite being mkvtoolnix.
For graphics you’ll find plenty of gif, jpeg and png (and other formats) on the web. For JPEG 2000, you can use some test samples at http://www.gwg.nga.mil/ntb/baseline/software/testfile/Jpeg2000/index.htm.
I’ll add more resources in the comments section as I found them. Please feel free to do so as well.