mirb is an small fast embeddable Ruby VM. It is designed to be simple to use and embed, while stil having a good performance due to a JIT-compiler and native C++ code. It may not end up featuring all of Ruby's core library because of the embeddable nature, but what is added should remain somewhat compitable.
It's still very much work in process and the only useful thing it can run (that I know of) is the included testing framework. docs/Todo.txt has a list of some essential missing features.
If you want to know more you can visit mirb.heroku.com or join me in #mirb on freenode.
You can either build it with rake (using Ruby 1.9) or with Visual Studio. Building with rake requires reno to be checked out in ./../reno. It also requires G++ of version 4.5 or greater. If G++'s executable is not called g++, you can override it with the environment variable CXX. You can also build it with Code::Blocks if you compile the assembly files for it.
Inspired by tinyrb, github.com/macournoyer/tinyrb