There really is no "target" audience of this post. :) It just started out as a list of "recommended" tools, then people started suggesting stuff -- some of which I knew, some of which I don't, some of which run on every unix, some only on Linux.
And it was easiest to take the screenshots from my OS X laptop. For some tools, like powertop, I had SSH-d into various Linux or FreeBSD boxes.
Some of these are uncommon enough that they are not ported to every package manager system, too. :(