The popularity of P2P file sharing of all sorts of video material has resulted in some people amassing large collections of files in every imaginable format. It is possible to rationalize the various formats in such a way that any arbitrary collection (within size constraints, of course!) can be combined into a DVD image that is playable on commercial hardware players.
Various example scripts are available which demonstrate how to deal with the bewildering variety of source formats:
[all2mpg] - A python script by Geoffrey Clements
[AviToVob] - An early shell script by Phil Ehrens
Mp42dvd - newer shell script based on Transcode-1.0.5 works with avi, mkv, ogm, mpg, mp4, mov