I benchmarked some tests run in JRuby that compare the Hamsterdam library using Hamster's immutable Hashes vs Clojure's PeristentHashMap, plus some gotchas.
Email clients all seem to display HTML mails differently. Here are a few tools I've used to make implementing Rails email templates a bit less painful.
RubyFlux statically compiles Ruby into Java. It has awesome performance and runs everywhere Java does with no runtime dependencies, but it isn't done yet.