Today Phusion has announced the release of Ruby Enterprise Edition (REE) 1.8.7 (more specifically, 1.8.7-20090928). Once considered a bit of a joke, given the name, REE has proven itself to be anything but, with significant memory usage and speed improvements over the stock "MRI" Ruby implementation (achieved by way of patches to the MRI code). The key development with this release is compatibility with Ruby 1.8.7, rather than the 1.8.6 of previous versions.
While REE has shown itself to be a good performer compared to the unpatched MRI implementation before, this week Evan Weaver of Twitter revealed how a release candidate version of REE 1.8.7 has resulted in a significant throughput increase on the same codebase. He also noted that Ruby is faster when compiled with optimization for size rather than speed due to the benefits of being able to fit more of Ruby into the CPU's cache. REE 1.8.7 is optimized for size by default, so you get the same improvements out of the box. For information on other optimizations and changes from MRI, Phusion's release post has more details.
Installing REE is pretty easy (instructions here) but the latest version of RVM (Ruby Version Manager) already has support for it if you want to seamlessly run multiple versions of Ruby.
Disclaimer: The REE "logo" shown above is not official and was designed by Ahmad Galal. It's CC 3.0 licensed.
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