Extra projects are those things that so many of us work on outside of our job. I've always thought of these activities as the one's that truly define a person. When my passions/interests and my job are not aligned, I think of what I love to do as my primary project(s) and my job as an extra project.
This prompted me to re-read The Hacker Ethic. Extra projects are the hacker ethic in action.
I hadn't seen the Hacker Ethic book before, I'll have to try to find it (Though I did read Linus' bio, does that count?). I've always had extra projects since I can't remember. There are many days when I can't imagine not having lots of pots on the burners, but there are others when I wonder why the hell I'm always doing other stuff and not out enjoying the day or relaxing and doing nothing, or excercising or whatever. I especially feel like this when I'm not producing anything, but spend the same amount of time "playing"... a real waste of time sometimes.
That's not normally the case, actually, especially with technology. Like I've told my wife (and this is true) there's not a single technology that I've ever started messing around with in my spare time that I eventually didn't use for work or even to get a new job. Linux and Java for me were like that... I was doing MS VB work until I decided one day to install a Linux box at home then started looking at Java as a way to program for it... the rest is history. All this mobile stuff that I'm obsessed about now is the same thing. Blogging too. Hopefully it'll come back around for me.
Some Spanish, by the way, definitely have side projects. Check out Blogalia as a great example. But I think most others go home and never look at a computer again and are aghast at the thought of it. They don't even care about the sector as a whole. Like that time last summer when my coworkers didn't know who Steve Jobs was. AAAH! Incredible.