Any developer or hobbiest who has ever written a line of code knows of StackExchange or StackOverflow!
Having begun my programming career many years ago I initially created an account to get some help on a few of my own questions, but soon found the value in finding random questions and trying to help other people solve them. Having begun this venture in college, this became an invaluable experience as it exposed me to a number of different software technologies being used in the real world. Helping users debug their problems in pieces of software I had barely used myself helped me to gain a better understanding on the inner-workings of these applications, many of which I still run to this day.
Being a site driven entirely by member contributions and moderation, my role has been insignificant however notable. In the 9+ years since I first registered an account on StackOverflow, I have accumulated over 25k reputation and metrics show that my posts have reached over 2.9 million people.
While neither number may seem impressive compared to others members of the community, with 25k reputation a user is granted all site privileges. I guess 2.9m is a lot of people too.