Erlang. Same here. Reading first few pages of a book describing principles of OTP (processes, share-nothing, messages, etc) was mind blowing. Company I worked for at the time (and I still do) decided to switch from Java to Erlang in middleware area. This decision seemed like a mixture of insanity and enlightment. Do you switch from one of the most popular languages in the world to something that most developers never heard of? Surely, exciting, but will it work? How do we hire new staff? After our R&D confirmed it was promising, me with couple of other developers were tasked with rewriting quite an expensive piece of middleware software that was unfortunately reaching its maximum capacity. We had no knowledge of how the software worked, we just knew its API. We were given time to learn erlang so we did. We all switched from eclipse to vim (some to emacs). After a bit of playing around with erlang we did our job in just 3 months. New app was much smaller and was easily capable of handling many more messages than the previous one. And it was written by erlang newbies! Then many more erlang apps we have created. It turned out to be a really good choice. Also the level of introspection you get out-of-the box with erlang is just amazing. I have never seen anything like this before.

Now I can compare Erlang to Java and it is really baffling how the heck Java took over the world. To do erlang I just need an editor with some plugins, ssh connection to linux with OTP installed and of course rebar3. To do Java I need 4GB of RAM to simply run an IDE with gazillion of plugins, maven to cater for thousands of dependencies for the simplest app and I need to know Spring, Hibernate, AOP, MVC and quite a chunk of other 26^3 3-letter abbreviations. No thanks.