My first real experiences with software engineering came when a high school friend introduced me to the Linux operating system, which came with full source code. At the time, I had done some medium-scale programming in HyperCard and Pascal, and was just beginning to study ANSI C.
The availability of complete GNU/Linux sourcecode gave me a chance to study any a complete, modern computer system; I was fascinated. Suddenly, my programming didn't have to be the creatin of tiny, private toys anymore. I could (after I had learned enough) make a difference by contributing to a real system that was in actual widespread (okay, perhaps a stretch at the time) use.
My first real involvement with a serious opensource project as a developer (instead of a user) came after netscape released the source to what would have been Netscape Navigator 5, creating the mozilla project. At the time, Netscape 4 was the only serious web browser available for UNIX machines, and was closed-source and quite buggy. The netscape source code release, while incomplete, provided a starting point and a focus to the efforts to create an opensource web browser. It also provided the strong incentive of creation to the developers joining to help the project; whatever they created would be released to a large userbase as Netscape 6.
It worked - mozilla worked on PowerPC. I built packages (Milestone 13 if memory serves), placed them up for download on my site (southpole.penguinppc.org), then on the official mozilla site, and had my first real success as I watched the downloads count up (over a quarter of a million downloads of Mozilla M13 - about 1200 of that first PowerPC build). I was hooked; this was doing it 'for real'!