There is a website, and probably an app, called Bible Hub, which has a plethora of resources including commentaries, sermons, all of the leading English translations, as well as translations in other languages.
Also, e-Sword Bible software is free and excellent: http://www.e-sword.net/. I've used it for over a decade. There are a TON of free or quite inexpensive biblically-faithful commentaries and reference works available for it. Most of the modules are older books, which I prefer. Check the "Links" page on the e-Sword link I provided for the gold mine -- many are Reformed or Puritan works.