I have a few of games I remember playing on a PS1 demo disc a long time back, and they were incredibly fun.
One was a first person shooter of some kind where you control a flying robot, and you can turn and rotate in any direction which made things a little disorienting. I remember what little I played involved a lot of corridors separating open rooms. This game I actually owned the full game for a while, but have lost the disc in a clearup a looooong time ago. I found this game also, it's called Forsaken, and I highly recommend it, although it is pretty challenging to control.
Another is a top down fighting kind of game, and all I really remember was the enemy using attacks that made crystals appear under me and blasting me about. Also the only level I could play on was a grassy area with small cliffs scattered around. I've found out this game, it's called Destrega, made by KOEI
Thirdly, an RTS game which I believe was only named by a number, it looked like a year like 2140 or something. What I remember involved leading a small collection of units across rocky terrain, and I had three engineer-type units that couldn't attack until you got to a weird pad on the ground which supplied them with mechs.
The forth I'm not to sure was an actual game, or just a bonus minigame included on the disc, but it involved shooting zombie-like babies in a small arena and I remember it having powerups and all kinds of things. This one I'm not too bothered about discovering, but it might help anyone who has played the same demo discs as me xD
As far as I recall, all these games appeared on the same demo disc but don't take my word for it, and the games had a four minute time limit to play them in.