In my own side projects, I'm often dealing with old, badly-documented formats with limited examples (like data files for games). I usually start with the "file" command to try to identify the filetype, then look on Wikipedia and filext.com to find links to format specifications. Usually, I can also find the name of any programs that create/edit/view that file type, and that's a jumping-off point to find examples (given the domain, it'll be anything from another game that uses the format to a 90s-era fan page with modified or fan-created data files).
I've used this site before too:
It provides links to a lot of format specifications, codec information, sometimes the mplayer samples that other comments here have linked to, etc.