Warp

This was the biggest game around at the time (early 80s). Bigger than Zork, way bigger than Colossal Cave. Bill Lucke and Luke Frolik wrote a parser arguably better than Infocom's and built a huge game. Unfortunately the game was written for HP3000 series minicomputers. The game was never officially released, but found widespread distribution through the HP INTEREX user community. It was huge. It was not always original (if the authors found a really clever puzzle in a different game they sometimes stole it), but there was a huge amount of original content. I lost hundreds of hours to this game in an MIT computer Lab in the early 80's.

I haven't seen this game available anywhere in 30 years, until this year's re-release but it was amazing.