I'm gonna go back a short bit and say, for me? Ruby on Rails.
In 2007 I was plugging away at PHP with whatever frameworks were around at the time -- the vast majority of web dev that I saw then was pure procedural scripting down the page, maybe some `include` statements to pull in database functions. Especially in PHP, which encouraged mixing logic and HTML. The consultancy I was with had built custom stuff on top of Zend framework, and it helped a lot.
But around summer of 2007 is when I started hearing folks crow about Ruby on Rails, this hot new web framework written by a Dane in a Japanese programming language. It had been out for a few years and it was the hot topic in web dev circles, and so I decided to see what the fuss was about one weekend.
Instantly stuff I'd always had to do by hand was done for me. A decent data access layer with a few lines of code per db table, that automatically handled preparing and executing statements, and could handle keeping the database up to date with migrations. A REPL where I could load up the code I'd written for the app and use it for one-off debugging or maintenance tasks. A thriving plugin scene where many of the things I'd bashed together over the years were available, for free, just by cloning a repo.
That weekend I reimplemented the core functionality of one of the apps we'd been working on for a client. We'd taken about 10 weeks to get this thing into rough shape and I had its equivalent in two not-very-busy days. Authentication, authorization, CRUD, and fancy database queries I'd all had to hand-roll before just fell out of Rails.
I'm not overselling it when I say that for me, Rails was an absolute game-changer, and I myself and many folks I know owe their careers to the Ruby community and its (not always perfect) attitudes about software development. MINASWAN.
(I'm still active in Rails and the local Ruby community [Columbus Ruby Brigade!] but I'm running an Elixir/Phoenix shop now, which feels to me like the next step on the path Rails forged us.)