Amy Hoy has written a great tutorial running through the Rails console that's suitable for both complete novices and experts (there are a couple of tricks she mentions that I didn't know at least!). Amy appears to have taken up Slash7 with a new zeal lately, so let's hope we'll see more great content like this on her blog through 2007!
"How To Think Like A Computer Scientist" is a range of online books (distributed under the GNU Free Documentation License) developed by various authors each focusing on a different programming language and explaining computer science concepts with it. Elizabeth Wiethoff has now started the Ruby edition and has made her work so far available online.
If you haven't got tab-completion and syntax coloring in your irb, you owe it to yourself to follow these instructions right away (should work for Linux, OS X, and Cygwin users). First, install the Wirble gem: