I'll keep it brief.
To the extend that it can be considered a hobby, I work on random programming projects (more recent ones are featured in the projects section). My projects have taken me through numerous languages; from PHP and Python to OCaml to Java. I still have a tendency to write larger programs in an object oriented style, and some would claim that I over-engineer smaller projects into object-oriented style as well, though I contest this in some of the cases.
In the vein of software, I also find my Linux machines (I only run Linux now, so that's all of them) somewhat of a hobby. Gentoo has its quirks which are sometimes fun to play with. Despite some problems, I can't live without my terminal, and cygwin is hardly a substitute when you know what you're missing; and my experiences with Macs have not been great.
In addition to software, hardware is also of mild interest to me, though mostly in terms of building computers, not really the designing circuit boards (though some of my research has looked at this problem). It's quite a bit more of an expensive hobby than software, so mostly it comes around when something breaks like in my recent move to Cambridge.
I enjoy running, hiking, cooking, and playing Go, though only the cooking has been happening much lately (since I have to eat). Lacking a car, hiking can be a little difficult, especially since to me hiking implies mountains and mountains in Boston aren't easy to come by. The trail by the river, however, has proven a good run. As far as Go, last I played, embarrassingly long ago now, I was about a 10kyu on KGS, my current lacking of a good Linux interface gets in the way a bit; but I still do problems which are enjoyable though probably not enough to actually be getting better.