Most Ruby developers should be familiar with Rack, an interface / library that abstracts away a lot of the banalities of hooking up application code to HTTP servers. It's used by several Ruby Web application frameworks already, some as a default - such as Merb, and others as an optional extra - such as Rails. Rack is significant because it provides a standard for Web-facing Ruby applications and frameworks to adhere to and is rapidly becoming the de-facto standard in this space.

Nick Sieger has developed JRuby-Rack, a variant of Rack that runs on the Java-based Ruby implementation, JRuby. You can use it to run any Rack-compatible application (e.g. Rails or Merb applications) inside a Java application server. This is significant because it gives application developers an independent, abstracted way to roll out Ruby-based Web applications on a Java server with significant ease (at least, more ease than before!).
If JRuby's of particular interest to you, you might also want to check out JRuby Inside, one of Ruby Inside's sister sites, and one dedicated entirely to JRuby links and news.

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