Came up against this recently when trying to upgrade a Rails app which was using MongoDB. Obviously I didn't want to use ActiveRecord, so I set about removing it.

The old way of removing ActiveRecord was to place this line in the environment.rb file, "config.frameworks -= [ :active_record ]". However, in Rails 3 you can simply pass the "-O" switch to the rails command, like so:-

 rails yourapp -O

But what if you've already created your app? If you look in your config/application.rb file, you'll see this line at the top:-

 require 'rails/all'

This does what it says on the tin; includes all rails modules. If you want to exclude ActiveRecord, remove the above line and replace it with:-

 # Pick the frameworks you want:
 # require "active_record/railtie"
 require "action_controller/railtie"
 require "action_mailer/railtie"
 require "active_resource/railtie"
 require "rails/test_unit/railtie"

This is the beauty of Rails 3, pick and choose what suits you.