In https://getprobe.io (built in Ruby) we use:

AWS (RDS, S3, ECS, EC2)

https://www.mailgun.com for emails

https://rollbar.com for error reporting

https://stripe.com for payments

https://gsuite.google.com for email, calendar, sheets

https://slack.com (free) for chat

https://trello.com (free) for keeping track of things