Ruby is something like a "improved" Python, with a better OO system, a code block syntax that makes it easy to use callbacks, more consistent standard libraries, etc. It could be what Python is today.

I wouldn't say niche, but the killer app of Ruby is Rails, a web framework similar to Django. In fact, many people treat them as they are the same. But there are big projects that use Ruby and that are not related to Rails. As far as I remember: Metasploit, Homebrew, Vagrant and Jekyll.

Personally I think Ruby is amazing language for writing shell scripts. I write a blog post about it, you can see it and its discussion here: https://news.ycombinator.com/item?id=40763640