{ |one, step, back| }
Running Back Versions of Gem-installed Applications
04 Mar 05 - http://onestepback.org/index.cgi/Tech/Ruby/Gems/RunnnigOldVersions.rdoc
I keep forgetting that RubyGems has this capability. In this example, I have 7 (yes, seven, really) versions of rake installed. 
  $ gem list --local rake

  *** LOCAL GEMS ***

  rake (0.4.15.1, 0.4.15, 0.4.14, 0.4.13, 0.4.12.1, 0.4.12, 0.4.4)
      Ruby based make-like utility.
  $
  $ rake --version
  rake, version 0.4.15.1
  $
  $ rake _0.4.12_ --version
  rake, version 0.4.12

Since rake is installed as a gem, you can run the executable of any installed version. By default, gems will run the latest one. But if you wish to run an older version, gems will accomodate you.