Article mentions lsof but doesn't go too deeply into it. lsof has an absolute _plethora_ of options and has been pretty indispensable to me. i.e. lsof -i (much akin to netstat), -u <user> (all files open by a particular user), etc etc. Check the man page :)
If you haven't seen Brendan Gregg's Linux performance tools page & slides, you should check it out. He explores a wide variety of great stuff, especially perf and ftrace (as mentioned in the article) as well as a few dozen others that he describes in lesser detail: http://www.brendangregg.com/linuxperf.html
I will just mention Sysdig for the ones who don't know about it already (http://www.sysdig.org/). It's really the swiss knife of monitoring tools, you can have any kind of information you can think of. If you have not already installed it, give it a try !
And contains more information, too. Before ss, it was close to impossible to know what the other end of a unix socket was for a given connection (except wild guesses from the ids in /proc/pid/fd).
vmstat [1] is also a very useful command. It shows various essential statistics in a very compact one line display and optionally repeats this every [interval]. I often start something running while vmstat is giving me stats in another window every few seconds.
pidstat! It's like top, except it prints a continuous log of everything that used CPU (or I/O with -d, etc.) since the last sample. It's the only too, I know that can show per-process I/O activity. Requires root access, unlike top.