Rails draws the attention, but there's plenty other uses.
Is there much happening in terms of libraries outside the rails scope or is it more a case of one less thing to worry about once you've become good with the batteries included?
Capybara is fantastic for automated testing
https://github.com/teamcapybara/capybara.
Gosu for 2d Games:
Example 2d game with Gosu:
https://github.com/victords/super-bombinhas
CRuby will be in browsers!
PS: https://github.com/shadawck/awesome-cli-frameworks also lists some for go and rust, getting there! (though most probably not half as complete as Thor, self-documentation should definitely be a first-class citizen!)
E.g. for ORMs ActiveRecord which is Rails default is in my opinions nowhere near good enough compared to e.g. Sequel, and there are several others.
Sometimes those projects involve wrapping stuff - including Python - or porting Python, but most of what I use is pure Ruby.
With respect to Python, to a lot of the Ruby community Python is in many ways the antithesis of what we want in a language. Python looks horrendous to me. It matters - I stare at code most of my day. If I were to switch languages, Python wouldn't make the shortlist even. It's not that I think Python is objectively worse, but subjectively to me it's unreadable and messy and I can afford to avoid using it most of the time.
EDIT: To expand on some of what I use or have used Ruby for: web dev with Sinatra and Padrino (my main reason for avoiding Rails here is that most of what I do is mostly API driven, and because I prefer Sequel as the ORM), financial modelling (I work for a VC), my text editor of the last few years is written in Ruby (but not packaged up to be useful for others yet - I'm gradually splitting parts of it out into gems, though), DevOps by generating terraform, service orchestration (proprietary pre-Kubernetes orchestration of containers and vms across on-prem, colocated, managed servers and VPSs split between openvz, docker and KVM), system to auto-deploy replicated sets of Postgres servers with logs and backup dumps automatically distributed to a Ruby storage service (pre-RDS, and running on the aforementioned orchestrator), PDF generation, messaging middleware, scraping, large scale crawlers, trading bots. And many more.
Rails has a clear one way to do things because there are no/few other options because everything else has withered away.
To put this another way: to have a convention in JS, just pick a convention, and now at least you have a convention, even if everyone else has their own.
Roda[1], for instance, has a strong following for API work.
It’s just that Rails is a safe choice. It’ll will work fine for small teams and it will scale to the size of github when/if you need it.