The official Ruby Web site contains an introduction to Ruby, the Ruby Application Archive, and pointers to more information.
Programming Ruby, The Pragmatic Programmer's Guide, by David Thomas and Andrew Hunt, is a well-written and practical introduction to Ruby. Its Web page also contains a wealth of Ruby information. Though the book is available online, I encourage you to purchase a copy.
A description of the ICE specification can be found on the ICE Standard Web Page, as can the ICE Implementation Cookbook: Getting Started with Web Syndication.
Ruby: An Introduction is a talk I have given a few times. You can find it online as HTML and PDF on my Ruby talk page. .
Have I mentioned the Rice home page yet? I have? Never mind.