One quite interesting use of Ruby outside of Rails was its inclusion as the scripting language in various versions of RPG Maker, a popular piece of game creation software. The developers of the software are Japanese, like Yukihiro Matsumoto, and I gather Ruby is used in broader situations in Japan than Rails.

I'm pretty sure most of Stripe is non-Rails Ruby, for one prominent example.

It's fairly popular for Mac. CocoaPods and homebrew are written in ruby

I see Ruby used pretty frequently in small parts of larger systems. I looked at a customer’s project last week and they were using Ruby to parse incoming e-mail.

The other somewhat significant use of Ruby that you may have heard of is Puppet. Much less in vogue for infrastructure as code these days but still useful for e.g. managing corporate fleets of Linux laptops.

BestBuy is a big Ruby shop.