Useful gems

Image/document uploads

• Paperclip
• Carrierwave

Sign in systems

• Clearance
• Devise

Payment processing

• Stripe
• ActiveMerchant
• money-rails

Styling and HTML

• bootstrap-sass
• zurb-foundation
• neat
• bitters
• haml-rails
• simple_form
• font-awesome-sass
• autoprefixer-rails

Some others

• geocoder • convert addresses to GPS coordinates, work with latitude/longitude
• prawn • generate PDF files
• chronic • turn date/time text into actual DateTime objects
• twilio-ruby • send SMS messages from your app