Advocacy
I have an Apple laptop running GNU/Linux since about a month now, and apparently, this occasionally helps to advocate GNU/Linux.
When I'm on the train, on my way from or to work, I will usually be doing stuff on my laptop. Of course, when you're on the train, you're usually not alone; and the fact that I'm using Apple hardware isn't too common in itself. "Oh, are you a Mac user?" "No."
People don't expect that answer. Yes, this is Apple hardware, but I'm running GNU/Linux. If they're smart enough to realize that Mac isn't the same thing as PC, then they usually are smart enough to have heard about GNU/Linux, too (although they call it plain 'Linux'). In this one month, it has happened on two separate occasions now that I've given a small demo about how good, nice and useful GNU/Linux is to other people on the train. Complete strangers. Compare to the whole year I've had a GNU/Linux Pentium IV-based laptop, where literally nobody asked me a thing about GNU/Linux – not even people I know. Of course, because the train ride is pretty short, that demo can't take longer than about half an hour, but it still is enough to show them how good an alternative this GNU/Linux thing is.
Both of them apologised for taking up my time. I told them I don't care. In fact, I may be more interested in explaining this to them than they are in listening...