Without a doubt some of the oldest programs that can be found in the Unix ecosystem, e.g. grep, rsync, wc, tr and so many more. If you have ever done some shell

Without a doubt some of the oldest programs that can be found in the Unix ecosystem, e.g. grep, rsync, wc, tr and so many more. If you have ever done some shell scripting, you know how valuable and well conceived this programs are. So I think that they being used so often by such an enormous and heterogeneous user base so many years after they were originally conceived, says a lot about how influential they are. The nice thing about them is that they are open source so you can have a look at how they have been implemented and probably step up your C and systems programming game a lot.