If you use a recent Ruby (in my case 1.9.2p0) then the very useful, cool,
and excellent 'rio' gem [1] will not work.  This library no longer seems to
be maintained by its author, Christopher Kleckner, whose most recent
announcement to ruby-talk under the nickname 'rio4ruby' was at least two
years ago.

There are a couple of forks on github that solve this problem, for example
https://github.com/gatherdigital/rio

To get Rio working for me, this is what I did:

  git clone https://github.com/gatherdigital/rio.git
  cd rio
  gem build rio.gemspec
  sudo gem install ./rio-0.4.3.1.gem

I post this here in the hope that it helps someone to use this excellent
library (again).

Thanks to the people who maintain forks of this library.  Hopefully an
official maintainer will emerge, reflected in the online docs at [1].

Gavin

[1] See rio.rubyforge.org for overview