Johnson is a Ruby wrapper of the SpiderMonkey JavaScript interpreter. In practice, this means you can use Johnson to evaluate JavaScript statements within a Ruby program.
It works really nicely, but there is no released gem and building the development gem is a bit of a headache. The instructions in the readme at the Johnson Github site appear simple but have several prerequisites that must be fulfilled before the whole process works. Here's what I did on Leopard. I'll try recreating on Windows tomorrow.
[sudo] gem sources -a http://gems.github.com
[sudo] gem install jbarnette-johnson
Multiple errors occur - if you see gems missing (like hoe, for example) then install them.
[sudo] gem install jbarnette-johnson
Errors occur, probably due to a failure to compile the native gem. Usually something about a missing rake/extensiontask.
[sudo] gem install rake-compiler
[sudo] gem install jbarnette-johnson
Yet another error occurs - a missing Manifest.txt file. Download from Github at http://github.com/jbarnette/johnson/tree/master and put it into the directory in the error message.
[sudo] gem install jbarnette-johnson
Success!
[Apr. 4, 2009 - Fixed typo, changing "rake-compile" to "rake-compiler"]