Cenophobie has created a Ruby Cheatsheet available in PDF and PNG formats. It's a little old (not out of date) but I hadn't seen it before and it's quite good. It only covers the basic, but is a useful reminder of which methods you have at your disposal on the basic classes like Object, String, Array, and Hash. It also includes a list of the special variables, command line arguments, reserved words, expressions, and so forth.