In my case, I have been a map addict since my early childhood (I'm currently 32 year old). When I was around 4 years old, one of my most valued posessions was a very old (1952, I think) edition of the Hammond's World Atlas - specially the historic maps section.
My current cell phone has a GPS receiver. Or, maybe I should say it as it really is: My GPS receiver can also act as a cell phone. Up until last year, I had never paid for a cell phone, but I wanted bad to have a GPS - So, the best deal for me was to buy it on a single package: I have a Nokia N95 phone.
I am also an active member of several Free Software projects - Most notably, I am a Debian developer. I had heard, of course, about OSM - But it was not until last August, when we held DebConf8 in Mar del Plata, Argentina, that I started mapping.
I live in Mexico City. The city is in perfect shape for me to start working on it: Most of the central city is mapped (and well mapped, AFAICT). I live precisely on the southwestern edge of what is mapped - So I have started working on Ciudad Universitaria and the neighbouring areas. Of course, whenever I go out of town, I also trace my way.
So far, it's great fun. And a time sink!

Le agregue un par de callecitas al mapa de Guadalajara cuando estuve ahi, en julio. Yo soy del Defe... y he estado dándole de a poquitos, conforme puedo, al sur-oeste de esta querida ciudad. Pero un abrazo pa'l occidente!