Well, I used to have my Pentium 1 playing movies that I converted to mpeg1. The conversion was fast (using my Pentium 4), and the quality was acceptable... I was using windows 98 at the time.
These days I have been trying to get avi/divx and mp4/h264 to play in the same Pentium1, this time using DOS. quickviewpro can do it with divx but sound is out of sync. I have yet to try some more with mp4 but I don't have high hopes (even though I am willing to use like a 160x120 version of youtube videos).
The way I see it, there is little point in using any other "old codec" other than mpeg 1. Conversion is very fast with modern computers (you can download a HD movie, and convert it down to VCD quality in about half and hour and watch it in an old pc) and quality is acceptable for what it is. I believe if someone has a good graphics card it will be easier, in my case is a laptop that has only a 2mbs graphics card with no acceleration for video.
Thanks for lolo799 for the videos in smacker format. I will also try those. I am curious if smacker can produce better results. All in all, I think that for retro computing we need:
1. Small file size (old drives are small we can't waste a lot of space with videos)
2. Acceptable quality (we are not going fr HD of course)
3. Codecs that can be played using Pentium or 486 (I doubt a 386 can play movies). Although I have seen a 286 playing a fullscreen video on youtube (custom software made and it was not really all that great).
There is a problem of not having a website providing videos on those formats nowadays.