Rails

Avoid member and collection routes.
Use private instead of protected when defining controller methods.
Name date columns with _on suffixes.
Name datetime columns with _at suffixes.
Name time columns (referring to a time of day with no date) with _time suffixes.
Name initializers for their gem name.
Order ActiveRecord associations alphabetically by association type, then attribute name. Example.
Order ActiveRecord validations alphabetically by attribute name.
Order ActiveRecord associations above ActiveRecord validations.
Order controller contents: filters, public methods, private methods.
Order i18n translations alphabetically by key name.
Order model contents: constants, macros, public methods, private methods.
Put application-wide partials in the app/views/application directory.
Use def self.method, not the scope :method DSL.
Use the default render 'partial' syntax over render partial: 'partial'.
Use link_to for GET requests, and button_to for other HTTP verbs.
Use new-style validates :name, presence: true validations, and put all validations for a given column together. Example.

Migrations

Set an empty string as the default constraint for non-required string and text fields. Example.

Use the user's name in the From header and email in the Reply-To when delivering email on behalf of the app's users.