Available for your exploration are MRI versions from 1.8.2 to 1.9.3, Ruby Enterprise Edition, JRuby, and Rubinius. There's also a nightly update of the latest Ruby head snapshot, available via RVM (see below) as ruby-head.
The current Rubies are under the control of RVM, the Ruby Version Manager. To use a particular current Ruby, you point to it with RVM, like this:
$ rvm use ruby-1.9.1 # or rvm use 1.9.1
You can also just use a specific Ruby command from the commandline:
$ ruby-1.9.1-p378 -e "puts 'hi'"
To see all the currently available Rubies, give this command:
$ rvm list
To see all the Ruby and IRB binaries available, see /usr/local/bin.
There's a lot you can do with RVM. For example, to run benchmarks on a Ruby script with several Rubies, you can do this:
$ rvm 1.8.6,1.8.7,ree benchmark my_file.rb
For more RVM ideas, see the RVM home page.
Please do not install RVM in your own account. The RVM installation on ruby-versions.net is system-wide. We don't have room for everyone to install their own Rubies.
When running on the site, you may get error messages about not being able to write certain files. If these messages stop you from being able to do something, please report them. We know about most of the annoying but harmless ones and will try to make them stop.
Enjoy!