Overall computer maintenance
Our home has become a combuter maintenance factory for all of our relatives.
With combined effort of me and my father we have fixed many computer or at least diagnosed out where the problem was.
I have once builded a combuter from a scratch, and for last couple of years I have not faced a single error or blue screen which I haven't been able to fix. Let it be software or hardware I can usually find the problem.
Thought most of my experience comes from Windows-platforms, I have at least used Linux and Mac couple of times.