For the Dev.to tech stack we use:

• Puma as the web server
• PostgreSQL as the primary database
• Fastly for edge caching
• Cloudinary for image manipulation/serving
• Airbrake for error monitoring
• Timber for logging
• Delayed Job and Active Job for background workers
• Algolia for search
• Redcarpet and Rouge to parse Markdown
• Carrierwave, Fog and AWS S3 for image upload/storage
• InstantClick (a modified version) instead of Turbolinks to accelerate navigation
• Heroku for hosting
• Heroku scheduler for scheduled jobs
• Sendgrid for transactional mailing
• Mailchimp for marketing/outreach emails
• Figaro for app configuration
• CounterCulture to keep track of association counts (counter caches)
• Rolify for role management
• Pundit for authorization
• Service Workers to proxy traffic
• Preact for some of the frontend. See the Frontend Guide for more info
• Pusher for realtime communication between the application and users' browsers
• GitDocs for beautiful and SEO-friendly documentation
• Git for version control
• GitHub for hosting the source code and issue tracking

This list is non-exhaustive. If you see something that belongs here, feel free to add it.