What is mruby ?
mruby is a light weight implementation of the Ruby programming language.
So if you use ruby then mruby might be useful for you – because it can run ruby program in interpreter form or compile and execute in vm form, probably in less amount of time (faster execution) with less amount of resources (that’s what the light weight indicates) as compared to the standard Ruby (MRI).
Ruby is already popular in web application development (Ruby on Rails framework) but the lightweight and efficient version of ruby would probably explore new domains for ruby – e.g mobile platform (Actually, it has started – checkout the bottom notes), embedded devices etc.
There isn’t any official version of mruby available right now but you can checkout the github page for downloading the initial release.
Notes : Now, Ruby is one of the most popular programming languages, within a few months you might be able to write mobile apps for Android and iPhone, in ruby using mobiruby (built on the top of mruby).