A sitemap according to Wikipedia is "a list of pages of a web site accessible to crawlers or users." While they are completely optional, Google uses the sitemap on your site to learn about it's structure. This allows Google and other search engines to potentially increase crawling coverage. While you can build this yourself manually via XML Builder or handcrafting an XML file, I prefer using the sitemap_generator gem.

One of the new features in Rails 4 is the ability to suffix controller-wide information to ETags. At the controller class level, you can set data by passing a block to the new `etag` macro style method. The etag method can be called multiple times, with each subsequent call suffixing the information to the response's ETag for easy client-side caching.

A lot of Ruby developers are familiar with queues, such as DelayedJob and Resque. While these queues have been predominant the last couple of years, my tool of choice is Sidekiq, a Redis backed queue by Mike Perham. Sidekiq advertises that a single process can do the work of 20 Resque or DelayedJob processes. Another benefit of Sidekiq is the inclusion of extensions, which allows for testing and delayed mailers.

One of the biggest changes coming in Rails 4 is the extraction of certain features and moving them to gems. Moving forward, Active Record observers and Action Controller sweepers are going to be available in the rails-observer gem.

Rails 4 has introduced a queue interface, allowing you use ActiveSupport::Queue out of the box or use any queue engine that complies with the interface.