Unfortunately RubyGarden doesn't exist anymore. Here are a couple of links to make up for it (the one Kevin posted is not the shortest one anymore by the way):
The first quines in Ruby
s="s=%c%s%c; printf s,34,s,34,10%c"; printf s,34,s,34,10
ruby quine slightly smaller than python quine
_="_=%p;puts _%%_";puts _%_
shortest nozero [sic!] ruby quine
puts <<2*2,2
puts <<2*2,2
2
