Updating since these answers are slightly outdated (although some still apply).
Simple feature added in rails 2.3.4, db/seeds.rb
Provides a new rake task
rake db:seed
Good for populating common static records like states, countries, etc…
http://railscasts.com/episodes/179-seed-data
*Note that you can use fixtures if you had already created them to also populate with the db:seed task by putting the following in your seeds.rb file (from the railscast episode):
require 'active_record/fixtures'
Fixtures.create_fixtures("#{Rails.root}/test/fixtures", "operating_systems")
For Rails 3.x use ‘ActiveRecord::Fixtures’ instead of ‘Fixtures’ constant
require 'active_record/fixtures'
ActiveRecord::Fixtures.create_fixtures("#{Rails.root}/test/fixtures", "fixtures_file_name")