The samples.mplayerhq.hu server has many samples for many different types of codecs. Most are self explanatory. Here is short overview of the more unobvious ones:

• A-codecs: obscure audio codecs
• internets: video conferencing codec samples
• karaoke: just midi files and some text
• fuzz and ffmpeg-bugs: files known to generate bugs in the two programs

The samples can be found on ​http://samples.mplayerhq.hu, ​http://samples.ffmpeg.org, or ​http://ghostarchive.org/samples/