What values for RUBY_ENGINE correspond to which Ruby implementations?

The method I know of to detect the Ruby implementation (e.g., MRI, JRuby, Rubinius, etc.) at run time is to check the global constant RUBY_ENGINE:

$ ruby -e 'puts RUBY_ENGINE'
ruby

What's a reasonably comprehensive list of known Ruby implementations and their corresponding values for RUBY_ENGINE?

Here's what I've found so far:

| RUBY_ENGINE | Implementation    |
|:-----------:|:------------------|
| <undefined> | MRI < 1.9         |
| 'ruby'      | MRI >= 1.9 or REE |
| 'jruby'     | JRuby             |
| 'macruby'   | MacRuby           |
| 'rbx'       | Rubinius          |
| 'maglev'    | MagLev            |
| 'ironruby'  | IronRuby          |
| 'cardinal'  | Cardinal          |

Links for the above mentioned Ruby implementations:

• MRI (Matz's Ruby Interpreter), the Ruby reference implementation
• REE (Ruby Enterprise Edition)
• JRuby
• MacRuby
• Rubinius
• MagLev
• IronRuby
• Cardinal

Props to Ujihisa for this blog entry which had most values. Others were found by reading this and this. Still more values have been gathered from other answers here on StackOverflow. More props to everyone who has contributed!