Rails 3.0 Release Notes – RailsGuides is an invaluable resource for all Ruby on Rails developers. The Rails 3 release notes provides a great overview the api changes and new features added to the framework.
RailsPlugins.org – Find out if your plugins work in Rails 3.
Having Ruby on Rails 3 My Way – A tutorial on how to setup Rails 3 with Mongoid, Haml, jQuery, and RSpec with Factory Girl.
The Rails Module (in Rails 3) – Nick Quaranto outlines some helpful methods in the Rails module.
Rails 3 I18n changes – A quick walkthrough the changes made to I18n.
Ruby on Rails 3 Screencast series – The official Ruby on Rails site screencast series by Gregg Pollack.
ActionController / Rack
Embracing REST with mind, body and soul – José Valim introduces RESTful responders within Rails 3 controllers.
Railscasts #224 Controllers in Rails 3 – Ryan shows us what’s new with Rails 3 controllers.
Render Options in Rails 3 – Yehuda goes through the rendering options available in Rails 3.
Adding Routes for tests / specs with Rails 3 – Jeff Dean outlines how to verify routes in our specs.
32 Rack Resources to Get You Started – A great collection of links to get you familiar with rack.
New ActionMailer API in Rails 3.0 – Mikel Lindsaar demonstrates how to use the new ActionMailer.
A Whole New ActionMailer – Mikel Lindsaar’s Rails Dispatch post giving a tour of the ActionMailer features.
ActiveModel: Make Any Ruby Object Feel Like ActiveRecord – Yehuda writes about the modularity of ActiveModel.
Railscasts #219 Active Model – See how to include certain ActiveModel modules to tableless models.
ActiveRelation: ActiveRecord Gets a Facelift – Carl Lerche introduces the new ActiveRecord finder api.
Active Record Query Interface – Pratik gives an overview of the changes made to the ActiveRecord query interface since Rails 2.
Gembundler.com -The official project site for Bundler that includes extensive documentation.
Library Management Gets an Update in Rails 3 – Andre Arko gives an overview about bundler for the Rails Dispatch blog.
Railscasts #201 Bundler – A Railscasts by Ryan Bates demonstrating how to use bundler with Rails 3.
AdventuresInCoding.com screencast on Bundler – Our very first screencast from last February.
Rails 3 Internals: Railtie & Creating Plugins – Ilya Grigorik goes over Railties in Rails 3.
rails-templater – A template project which generates a greenfield Rails 3 application.
Making generators for Rails 3 with Thor – A tutorial showing you how to use the new generators and how to bundle them in a gem.
The Lowdown on Routes in Rails 3 – Rizwan Reza gives us a great summary of all the different ways to create routes in Rails 3.
Rails Routing from the Outside In – RailsGuide documentation about the new router.
Plugin Authors: Toward a Better Future – Yehuda goes through the changes for making plugins in Rails 3.
Customizing Rails Apps with Plugins – José Valim gives an overview of using responders, simple form, devise, and mail form plugins.
Devise 1.1 is out and ready to rock with Rails 3 – Outlines some new feature in Devise 1.1 including being able to authenticate a given url in the router!
Beginning Rails 3 – Cloves Carneiro Jr and Rida Al Barazi bring us a great beginner Rails 3 book with examples viewable in gists.
The Rails 3 Way – The ultimate Ruby on Rails reference by Obie Fernandez.
Rails 3 In Action – Ryan Bigg and Yehuda Katz deliver a great Rails book which teaches readers Rails by doing BDD with Cucumber and RSpec.
Rails 3 Upgrade Handbook – Jeremy McAnally’s book gives you 120 pages of information about upgrading your Rails 2 application to version 3.
Ruby on Rails Tutorial: Learn Rails by Example – Michael Hartl’s ebook that teaches Rails step by step.
Update: Rails 3.0.0 was released on August 29th, 2010. To get more details, be sure to read the official announcement.