I recently looked into some alternatives to Rubymine, for doing some (Ruby) side projects. I mostly use Rubymine at work, so I thought it would be a good idea to familarize myself with another IDE. While I have used editors to work on Ruby, in particular Atom and Sublime and heard that Vim (plus plugins) is another popular choice, I wanted to see what is out there. To make it short how my search ended: I will use Atom, for the time being, when I work with Ruby. (First I will rather work on this blog and a project for which I like to use Jekyll, or another static site generator though.) The Ruby ecosystem seems to no longer have supported a variety of IDEs anymore in recent years. You do find quite a few discontinued projects though and others which no longer have full support.
Netbeans
“NOTE: As of NetBeans IDE 7.0, support for Ruby and Ruby on Rails is no longer available in the standard NetBeans IDE build. Please see the Ruby Support Wiki page for more information.” it says. And this Wiki page has last been edited in 2013! The plugin itself has last seen an update in 2015.
Ruby in Steel
This IDE by SaphireSteel Software seems to have been last updated in 2010. Maybe it was popular back then, because posts mentioning still rank highly in some of my searches. From some comments I gathered that it might have rather been a plugin to Visual Studio, at least at some point, not a fully independent project.
Redcar
Not actually an IDE, but as it is an editor written in Ruby, with working on Ruby in mind, among other things, it gets an “honorable mention” here. Last update of the repo was in May 2013.
Arcadia IDE
This is an actual Ruby IDE written in Ruby, under the Ruby license. However, the repo was last updated in 2015, and it looks like the bulk of the work was done by only two contributors.
eric
First and foremost a Python IDE, but Wikipedia mentions it as a Ruby IDE. On the website, I did not find any mention of Ruby though.
KUSO IDE
An Emacs-based project. Their Ruby support plugin last got an update in 2014.
Visual Ruby
Not a general purpose IDE, but rather a tool to create GUIs for Ruby, based on what I get from the site. Last updated in July 2016, so in comparison still somewhat “active”.
Aptana Studio
Aptana Studio seems to actually be still active in their Ruby support! I am not sure why I decided on not using it for now, actually. Maybe I will get back to it, once I work on something with Ruby.
Komodo IDE
Also seems to still have active Ruby support. A license costs significantly more than RubyMine though, so I do not think I will give it a try.
There might be a couple more I have not come across now, of course. I do not know how this compares to other languages, for example Python, and if it might be a common theme that for a fairly “mature” language there is more obsolete IDEs than current options. That does seem likely though, given the economics involved. Several Ruby IDEs seem to have been discontinued around 2013 till 2015. At that time, Rails was already at version 4 and fairly popular, so development work necessary to move from a general Ruby IDE to an IDE that specifically supports Rails probably was not a major factor.