Game demos.. Usually shareware, a earlier section of the game, huge sections of content missing since you didn't pay for it.... Doom, Wolfenstein, Jazz Jack Rabbit, Jill of the Jungle... Tyrian Episode one (that was fun), Warcraft... So many memories...

I'm remembering downloading/getting the Starcraft demo, it was like 30Megs back in the day (and Diablo 1 was 50). They were originally calling the alien species zurg, and the section you could play was effectively a tiny tutorial area, where likely only a few zurglings, buildings, and marines were even in the download and the rest stripped so you couldn't play it much anyways. Missing audio too...

Demos are effectively dead as mentioned before...

Actually there was a lot more demos on the Xbox 360, which was actually the full version but it wouldn't let you play beyond a certain point unless you 'paid' for it... So it was DRMed, but some versions of the 360 before certain updates would let you play the full game...

Demos aren't dead, just very rare these days. Steam offers demos if available (mostly for indies, but also for a few AAA titles) but I guess Valve can afford it and I could imagine many devs relying on Steam's service and not offering the demos elsewhere because of that. I think Desura hosts some demos, too, though. So the problem is not so much that demos don't exist, but that GOG doesn't have a system in place to allow hosting them. Still, it's true that if GOG would offer them, they'd probably be available only for a few indie titles.

Anyway, I updated the demo thread with a few newer entries, and for the older ones, also check out the GOGWiki.