I'll second that :) SphereServer for me but it's also what got me into programming and I have very fond memories of using Dragon, InsideUO etc. to create a new world. I'm guessing kids today would do something similar with Minecraft.

I also tinkered around with Sphere.

It was such a awesome feeling after years of playing the game as intended by the developers to just beam myself around the map and explore all the hidden stuff I never seen during playing this game as a normal player for years.

Same for me! My first exposure to programming was writing mining macros around Vesper.

Same here, but with UOX a few years before!