in the current commercial project i'm working on, we use a number of different open source libraries. it is very fortunate for us that we don't have to write everything ourselves or we would get nowhere fast. one of our libraries *used* to be closed source but it stopped being maintained so we switched it for an open source alternative. and it is just as well some of them are open source because i have actually had to fix some bugs in our dependencies.
Also we were investigating another closed-source library, only the closed-source libraries often tend to be Windows-only and we are aiming at multi-platform in the long term (perhaps Linux, which i'm currently using to develop it, but definitely Mac).
Of course anyone who uses C++ already uses Standard Library! The code i'm working on was originally written by people of less experience, and they had really not taken as much advantage of Standard Library as they could have. If it seems like something a lot of people might want to do, there is probably an algorithm or data structure for it there already!