I echo some of the sentiment that some of these gems provide, but they are quite helpful in getting a quick project up and running in a few minutes to start playing around.
Some of my favorites:
figaro[1] Alternative to dotenv, awesome mascot.
rack-mini-profiler[2] Often folks will say to save performance tuning until the end. I find having this around from the get go makes it much easier to do as you move along.
I love sidekiq though. Probably the best on the list. Background queues are essential to most, if not all Rails applications. The goworker post from today looks interesting too.