Xmlvalidation is a project mainly written in Ruby, it's free.
JAXP + Sinatra on JRuby
As far as i know no libxml based xml library can validate against arbitrary schema, only 1 specified schema. JAXP has pretty good support for any sort of XML Schema validation.
with jruby installed: % bundle exec ruby app.rb
open up http://localhost:4567, choose a file and click validate. that's it.
or use curl: % curl [email protected] localhost:4567
all (maybe most) the standard rack stuff applies, you are limited to webrick, mongrel or one of those java things which I don't know about.