Victor Grey and Kermit Snelson have created an XRI proxy resolver in Ruby called Barx. In it's most simple form, a proxy resolver returns an XRDS document when given an XRI. From the spec: "Proxy resolvers enable applications even those that do not natively understand XRIs but can process HTTP URIs---to easily access the functions of an XRI resolver remotely." An example is xri.net. Barx implements the entire XRI resolution spec with the exception of SAML trusted resolution.
According to Victor, "[t]he proxy resolver is a fast HTTP server based on Mongrel and Merb that can be run as a local service (on localhost:someport) to provide XRI resolution and caching for other applications such as OpenID relying parties, or as a scalable public service if desired." The Ruby gem can be installed with
gem install barx
and the proxy can be downloaded from Rubyforge.