4th option are Ruby things other than any of the official releases mentioned above...
JRuby (jruby) is an implementation of Ruby that can be run inside JVM (Java Virtual Machine).
Rubinius (rbx) is a Ruby interpreter with advanced features and technologies such as, LLVM, generational garbage collector and many more.
ruby-head, AFAIK, refers to HEAD commit of Ruby source code, eq. "bleeding edge" of the latest and greatest but might now work as expected since it represents a work in progress.
Hope that helps.