I'm pretty sure I had the very first demo disk they put out for the ps1. I can't remember what all was on it, but I know some games where playable and some demo's you just watched, but I would have to say my favorite was Wipeout.
I had a different demo disc that came with my PS1 in 1998. It was called Interactive Sampler 5 or 6 and it had Blasto, Einhander, Tomba, Tomb Raider 2, Gex 2, and Tekken 3 all in playable form. It also had trailers for Resident Evil 2, Metal Gear Solid, and Spyro The Dragon.
The demo disc I got had Tekken 2, Blast Chamber, Cool Boarders, Crash Bandicoot and some others that I can't think of. I played Tekken 2 daily until it released...then I played the actual game 24/7 lol