One of the big milestones in a ruby implementation is when it's able to first run a simple rails application. Until a couple of weeks ago, there were only two ruby implementations that were able to do this - the C reference implementation (Matz's ruby) and JRuby.
Today, there are four implementations that have hit this milestone.
Rubinius and
Iron Ruby have both just announced that they're able to run a simple rails app. This is an impressive milestone for both projects - congratulations to everyone working on those.