I made the classic 2 rooms connected by a hallway in Worldcraft for HL1. That was the first time I learned what a leak was. Compiling was hell back then. Man, when I moved onto making a button that opened a door, I thought I was Carmack.
I had plans to create a full single player mod - I wonder if I still have the plans for all that. It was essentially just fighting through many floors of a black mesa skyscraper.
After making many many test maps I eventually started working on HLDM maps and then DoD maps.
Who remembers the 30-60-90 minute map contests? I found a zip file full of hundreds of those the other day.