Rails

In general:

Prefer to commit your config/*.yml files, except database.yml (PRIVATE REPOS ONLY!)
Make use of special Rails ANNOTATIONS like TODO, FIXME, and OPTIMIZE so rake notes can pick it up.
Keep README.md updated (always assume a new team member will join anytime)

CSS and JS:

Server:

Style Guides:

Emails:

Rake tasks:

Ensure db:migrate runs properly
Ensure db:rollback runs properly
Ensure db:seed runs properly (They get outdated fast!). Alternatively, provide a way to seed a newcomer's project.
Ensure painless turnovers

Configuration:

Use config/secrets.yml to store config
Use 12-factor style when overrides are needed: pusher_id: <%= ENV['PUSHER_ID'] || '...' %>
Access them in your app via Rails.application.secrets.pusher_id

Avoid:

Avoid less-rails. Slow to update, and doesn't support import globbing
Avoid compass unless necessary. Use autoprefixer instead
Avoid bootstrap-on-rails because it's .less and not currently maintained
Avoid twitter-bootstrap-rails because it's .less and has too many abstractions
Avoid coffeescript. Prefer to use sprockets-es6 instead. (article)
Avoid bower-rails. Heroku setup can be a pain, and rails-assets is a better choice nowadays.
Avoid configuration gems like figaro, dotenv, etc. Rails 4.1 should render them useless now.
Avoid teaspoon. It has more boilerplate to setup, and doesn't sandbox your test DOM unlike konacha. Use konacha instead
Avoid Enums. They can cause non-obvious problems with queries (1, 2, 3). Use strings instead.

Also see