Web Based Session Management - Some great basic information about how to do sessions on the web
Rails Session Management
cheat for :has_one - From a great site that has a whole bunch of cheat sheets for Rails. Essential.
Developing a Rails model using BDD and RSpec, Part 1
Low Pro - Unobtrusive scripting for Prototype. Goes great with Rails.
Things you shouldn't be doing in Rails - I now use the excellent deprecated plugin which checks these all for you automatically. Highly recommended.
ActiveRecord Extensions - Very interesting, some extensions to ActiveRecord.
CSS Box model - Essential reading. Really.
Some really excellent Drawing Tutorials