bearcat33: I seem to recall seeing a link here on Gog that led to a Youtube "movie" demonstrating
a series of games....not exactly what I think of as a demo, but this really is essential.
Sure, at the bottom of
each and every page is a link to GOG.com's Youtube channel. I'll let you find the link yourself. They haven't made movies for every game they sell, but other people have probably made some and made them available on youtube so it shouldn't be too hard to find videos of most games (yes, that's most, I realise that probably not all games have been recorded and uploaded).
Take a look in the demo thread that I linked, it contains links to the original demos that the publishers made available so I cannot say wether they work or don't on your or anyone else's system. They have not been patched to the latest versions (how often do patches happen for demos anyway?), they are not bundled with DOSbox/ScummVM (for the cases where it matters), nor have the installers been replaced so some are probably 16-bit and thus won't run and/or install on 64-bit systems. But there you have it.
Demos.