When I was in middle school, I thought SC's trigger-based map editor was the most amazing thing ever. I even built a four-mission mini campaign that I showed my friends, who promptly told me that it sucked. (It was actually pretty terrible, although fun to make.) That, and collecting UMS maps and putting them on my GeoCities page.

The incredible part is, DotA started out as a Warcraft 3 custom map. Hell, there's still a version being maintained by the developers at Valve which largely maintains feature parity with DotA 2.

Oddly enough, battle.net itself is what got me into computers. There was a vibrant hacking community that existed back in the day, and watching all those older kids do cool stuff got me interested in software.